feat: derive the release bump from conventional commit subjects

The bump was decided by `[major]` / `[minor]` markers alone, so a range of
ordinary `fix:` commits and a range of `feat:` commits released the same way,
and the version number said nothing about what changed.

Read the intent from the subject instead, keeping the explicit markers as
overrides for the cases a subject cannot carry:

  `feat` as its own word   minor   0.1.0 -> 0.2.0
  `!:` or BREAKING         major   0.1.0 -> 1.0.0
  anything else            patch   0.1.0 -> 0.1.1

`feat` has to be a word, so "Fix the feature flag leak" is a patch rather than
a minor. Only the subject line is read; the body is not.

Adds `npm run release:notes`, which prints the notes a push would publish, and
AGENTS.md, which states the rules where an agent will read them.

AGENTS.md

@@ -0,0 +1,39 @@
+# Working in this repository
+
+`master` releases itself. Every push runs `.gitea/workflows/release.yml`, which
+reads the commit subjects since the last tag, bumps `package.json`, prepends
+`CHANGELOG.md`, tags `vX.Y.Z`, and publishes the Gitea release with those
+subjects as the body.
+
+## Commit subjects are release input
+
+The subject is both the version decision and the sentence a user reads in the
+release notes. Write it for that reader:
+
+- `feat: …` — the change adds a capability. Minor bump: `0.1.0` → `0.2.0`.
+- `feat!: …` or `fix(x)!: …` — the change breaks an existing install. Major
+  bump: `0.1.0` → `1.0.0`, and the note lands under **Breaking**. `[major]`
+  anywhere in the subject does the same when a `!` will not fit.
+- `fix: …`, `docs: …`, or a plain sentence — everything else. Patch bump:
+  `0.1.0` → `0.1.1`.
+- `[skip release]` — this push publishes nothing.
+
+`feat` has to be a word, so `Fix the feature flag leak` is a patch. Only the
+subject line is read; the body is not.
+
+## The release job owns the version and the changelog
+
+`package.json`'s `version` and `CHANGELOG.md` are the job's to write, derived
+from the commits. Pushing to `master` is the whole trigger — the tag and the
+release come with it.
+
+## Checking before pushing
+
+```sh
+npm run release:plan     # would this HEAD release? (yes/no)
+npm run release:notes    # the exact notes it would publish
+npm run test:release     # the checks behind the bump and the notes
+```
+
+The rules in full — the marker table, first-release behaviour, how prose commits
+are handled — are in the README under *Releases*.