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.

.gitea/scripts/release.test.sh

@@ -100,6 +100,41 @@ commit "Rewrite everything [major]"
 check "major outranks minor"      "major" "$(bash "$RELEASE" bump "v0.1.3..HEAD")"
 cd /;
 
+echo
+echo "the subject decides the bump"
+new_repo 0.1.0 >/dev/null
+tag v0.1.0
+check "prose is a patch"  "patch" "$(bash "$RELEASE" bump "v0.1.0..HEAD")"
+commit "feat: add the recall index"
+check "feat is a minor"   "minor" "$(bash "$RELEASE" bump "v0.1.0..HEAD")"
+check "so 0.1 -> 0.2"     "0.2.0" "$(bash "$RELEASE" next-version "$(bash "$RELEASE" bump "v0.1.0..HEAD")")"
+cd /;
+
+new_repo 0.1.0 >/dev/null
+tag v0.1.0
+commit "feat(scope): add the scope filter"
+check "a scoped feat counts" "minor" "$(bash "$RELEASE" bump "v0.1.0..HEAD")"
+cd /;
+
+new_repo 0.1.0 >/dev/null
+tag v0.1.0
+commit "Fix the feature flag leak"
+check "feat inside a word is not a feature" "no"    "$(bash "$RELEASE" is-feature "Fix the feature flag leak")"
+check "so it stays a patch"                 "patch" "$(bash "$RELEASE" bump "v0.1.0..HEAD")"
+cd /;
+
+new_repo 0.1.0 >/dev/null
+tag v0.1.0
+commit "feat!: move the index to sqlite"
+check "a bang is breaking" "yes"   "$(bash "$RELEASE" is-breaking "feat!: move the index to sqlite")"
+check "so it is a major"   "major" "$(bash "$RELEASE" bump "v0.1.0..HEAD")"
+check "and 0.1 -> 1.0"     "1.0.0" "$(bash "$RELEASE" next-version "$(bash "$RELEASE" bump "v0.1.0..HEAD")")"
+NOTES="$(bash "$RELEASE" notes "v0.1.0..HEAD")"
+check_contains "breaking gets a section" "### Breaking" "$NOTES"
+check_contains "and is listed there"     "- move the index to sqlite" "$NOTES"
+check_absent   "not under Added"         "### Added" "$NOTES"
+cd /;
+
 echo
 echo "skip release"
 new_repo 0.1.0 >/dev/null