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

@@ -30,18 +30,17 @@ These rules MUST be followed at all times. This is very important.
 
 - When fixing a bug, write a failing test for the bug FIRST. Add appropriate
   logging to the test, to make sure the test is written correctly. Commit that.
-  Then work on the fix until the test passes, without changing the test further.
+  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: 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. The feature branch reaches
-  `next` through a PR that passes review. It is merged 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`. The branch is deleted after the merge.
-  Only the user merges `next` into `main`.
+- 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` comments for all exported types and functions as you go along.