Rewrite AGENTS.md in short plain sentences (#327)

Every rule keeps its meaning and emphasis. Sentences are split so each
holds one idea, with the main rule before its exception and every
command, file name and identifier in backticks.

Three statements were false against the repo or the standing rulings
on record and are corrected:

- the format check is `make fmt-check`, not `make check-fmt`
- a feature branch is merged into `next`; only the user merges `next`
  into `main`
- the naming example uses the code's term: `al` is bad,
  `SourceHostAllowlist` is good

The `Last Updated` line takes today's date.

Model: fable-5-1

AGENTS.md

@@ -1,75 +1,75 @@
 # Repository Rules
 
-Last Updated 2026-01-08
+Last Updated 2026-10-27
 
-These rules MUST be followed at all times, it is very important.
+These rules MUST be followed at all times. This is very important.
 
-- Never use `git add -A` - add specific changes to a deliberate commit. A commit
-  should contain one change. After each change, make a commit with a good
-  one-line summary.
+- Never use `git add -A`. Stage specific changes for a deliberate commit. A
+  commit holds one change. After each change, make a commit with a good one-line
+  summary.
 
 - NEVER modify the linter config without asking first.
 
-- NEVER modify tests to exclude special cases or otherwise get them to pass
-  without asking first. In almost all cases, the code should be changed, NOT the
-  tests. If you think the test needs to be changed, make your case for that and
-  ask for permission to proceed, then stop. You need explicit user approval to
-  modify existing tests. A bug fix may change an existing test without asking
-  when that test pins exactly the behavior the filed bug calls wrong, and the
-  change moves it to the fixed behavior and nothing else; any other change to an
-  existing test still needs explicit user approval. (You do not need user
-  approval for writing NEW tests.)
+- NEVER modify a test to exclude special cases or otherwise make it pass without
+  asking first. In almost all cases, change the code, NOT the test. You need
+  explicit user approval to modify an existing test. If you think a test needs
+  to change, make your case and ask for permission to proceed. Then stop. A bug
+  fix may change an existing test without asking only when that test pins
+  exactly the behavior the filed bug calls wrong. The change must then move the
+  test to the fixed behavior and nothing else. Any other change to an existing
+  test needs explicit user approval. Writing NEW tests needs no approval.
 
-- When linting, assume the linter config is CORRECT, and that each item output
-  by the linter is something that legitimately needs fixing in the code.
+- When linting, assume the linter config is CORRECT. Treat each item the linter
+  reports as something that legitimately needs fixing in the code.
 
 - When running tests, use `make test`.
 
-- Before commits, run `make check`. This runs `make lint` and `make test` and
-  `make check-fmt`. Any issues discovered MUST be resolved before committing
-  unless explicitly told otherwise.
+- Before committing, run `make check`. It runs `make test`, `make lint` and
+  `make fmt-check`. Any issue it finds MUST be resolved before committing,
+  unless you are explicitly told otherwise.
 
 - When fixing a bug, write a failing test for the bug FIRST. Add appropriate
-  logging to the test to ensure it is written correctly. Commit that. Then go
-  about fixing the bug until the test passes (without modifying the test
-  further). Then commit that.
+  logging to the test, to make sure the test is written correctly. Commit that.
+  Then fix the bug until the test passes, without modifying the test further.
+  Then commit the fix.
 
-- When adding a new feature, do the same - implement a test first (TDD). It
-  doesn't have to be super complex. Commit the test, then commit the feature.
+- When adding a new feature, do the same: write a test first (test-driven
+  development, TDD). The test does not need to be very complex. Commit the test,
+  then commit the feature.
 
-- When adding a new feature, use a feature branch. When the feature is
-  completely finished and the code is up to standards (passes `make check`) then
-  and only then can the feature branch be merged into `main` and the branch
-  deleted.
+- When adding a new feature, use a feature branch. The feature branch is merged
+  into `next`, and then deleted, only when the feature is completely finished
+  and the code is up to standards. The code is up to standards when it passes
+  `make check`. Only the user merges `next` into `main`.
 
-- Write godoc documentation comments for all exported types and functions as you
-  go along.
+- Write `godoc` comments for all exported types and functions as you go along.
 
-- ALWAYS be consistent in naming. If you name something one thing in one place,
-  name it the EXACT SAME THING in another place.
+- ALWAYS be consistent in naming. If you name something one way in one place,
+  use the EXACT SAME name in every other place.
 
-- Be descriptive and specific in naming. `wl` is bad; `SourceHostWhitelist` is
+- Be descriptive and specific in naming. `al` is bad; `SourceHostAllowlist` is
   good. `ConnsPerHost` is bad; `MaxConnectionsPerHost` is good.
 
-- This is not prototype or teaching code - this is designed for production. Any
-  security issues (such as denial of service) or other web vulnerabilities are
-  P1 bugs and must be added to TODO.md at the top.
+- This is not prototype or teaching code. It is designed for production. Any
+  security issue (such as denial of service) or other web vulnerability is a P1
+  bug. It must be added at the top of `TODO.md`.
 
-- As this is production code, no stubbing of implementations unless specifically
-  instructed. We need working implementations.
+- Do not stub an implementation unless you are specifically instructed to. This
+  is production code, and we need working implementations.
 
 - NEVER silently fall back to a different setting when a user's parameter
-  explicitly specifies a value. If a user requests format=webp and WebP encoding
-  is not supported, return an error - do NOT silently output PNG instead. If a
-  user specifies fit=invalid and that fit mode doesn't exist, return an error -
-  do NOT silently default to "cover". Silent fallbacks violate the principle of
-  least surprise and mask bugs. The only acceptable defaults are for OMITTED
-  parameters, never for INVALID explicit values.
+  explicitly specifies a value. For example, if a user requests `format=webp`
+  and WebP encoding is not supported, return an error. Do NOT silently output
+  PNG instead. If a user specifies `fit=invalid` and that fit mode does not
+  exist, return an error. Do NOT silently default to `cover`. A silent fallback
+  violates the principle of least surprise and masks bugs. Defaults are
+  acceptable only for OMITTED parameters, never for INVALID explicit values.
 
-- Avoid vendoring deps unless specifically instructed to. NEVER commit the
-  vendor directory, NEVER commit compiled binaries. If these directories or
-  files exist, add them to .gitignore (and commit the .gitignore) if they are
-  not already in there. Keep the entire git repository (with history) small -
-  under 20MiB, unless you specifically must commit larger files (e.g. test
-  fixture example media files). Only OUR source code and immediately supporting
-  files (such as test examples) goes into the repo/history.
+- Avoid vendoring dependencies unless specifically instructed to. NEVER commit
+  the `vendor/` directory. NEVER commit compiled binaries. If such a directory
+  or file exists and `.gitignore` does not list it, add it to `.gitignore`.
+  Commit that `.gitignore` change. Keep the entire git repository, history
+  included, under 20 MiB. Go over 20 MiB only when you specifically must commit
+  a large file, such as a media file used as a test fixture. Only OUR source
+  code and its immediately supporting files, such as test examples, go into the
+  repository and its history.