Rewrite AGENTS.md sentence by sentence

Rewrite the agent rules from scratch, as a newcomer would read them,
with one idea per sentence and every file name and command in
backticks. Drop the rule that models embed `models.Base`, which no
package in the repository has, and the statement that tests share
their package. Nearly every test is in an external test package.
Where a test needs an unexported name, an `export_test.go` file in
the package exposes it. State the branch workflow in use: pull
requests target `next` and only the owner merges `next` into `main`.
Record the three snake_case JSON tags the HTTP API keeps.

Part of #190.

Model: fable-5-1 (writing); opus-5-5 (rework)

AGENTS.md

@@ -10,8 +10,8 @@ shared documents and are not edited here.
 2. Run `make check`. It runs the tests and the linter in Docker, then the format
    check. All three must pass with no findings.
 
-Never run `go test`, `golangci-lint` or `gofmt` directly. The `make` targets
-hold the flags and the Docker setup.
+Never run `go test`, `golangci-lint` or `gofmt` directly. The scripts in
+`script/` hold the flags and the Docker setup.
 
 ## Branches and pull requests
 
@@ -38,14 +38,14 @@ hold the flags and the Docker setup.
 
 ## Architecture
 
-- Dependency injection uses Uber `fx`. Every component is an `fx` provider,
-  wired together in `cmd/neoircd/main.go`.
+- The server uses `go.uber.org/fx` for dependency injection. Each of its
+  components is an `fx` provider, wired together in `cmd/neoircd/main.go`.
 - Application packages live under `internal/`, one package per concern.
   `pkg/irc` holds the IRC command names and numeric reply codes.
 - A handler method returns an `http.HandlerFunc`.
 - A request struct is defined inside the handler that decodes it.
 - Database access goes through the methods of `db.Database`.
-- The database schema lives in `internal/db/schema/` and is embedded with
+- The database migrations live in `internal/db/schema/` and are embedded with
   `go:embed`.
 - The database is SQLite, through the pure Go driver `modernc.org/sqlite`.
 
@@ -54,11 +54,12 @@ hold the flags and the Docker setup.
 - Prefer table-driven tests.
 - Use `testing.T` helpers, not assertion libraries.
 - A test file lives next to the code it tests and ends in `_test.go`.
-- Tests are in the package's external test package, such as `handlers_test`. An
-  `export_test.go` file in the package itself exposes what the tests need.
+- Tests are in the package's external test package, such as `handlers_test`.
+  When a test needs an unexported name, an `export_test.go` file in the package
+  exposes it.
 
 ## Dependencies
 
 - The required libraries are listed in `CONVENTIONS.md` §1.
 - No new dependency without a reason given in the pull request.
-- Prefer pure Go. No cgo.
+- Prefer dependencies written in pure Go, which build without cgo.