ci: dogfood with conventional-commits and document release conventions

[lesnik512]

Two small maintenance changes.

Dogfood with conventional-commits. semvertag.yml ran the branch-prefix strategy, which needs a merge commit at the head of main. This repo squash-merges, so every run reported no_merge_commit and never exercised a bump path (e.g. the run on d333e65). The squash titles already follow Conventional Commits; running the strategy over the last 25 commits on main gives feat: → minor, fix(deps): → patch, and docs:/ci:/test:/refactor:/chore: → none, as expected. A local dry run of this branch against the repo with conventional-commits reports already_tagged 0.9.0 for the current head. The now-unused SEMVERTAG_BRANCH_PREFIX__MINOR override is dropped, and the header comment says the job computes the head commit's bump: per ADR-0006 it judges one commit, not what has accumulated since the last release.

Release conventions in AGENTS.md. The existing "Cutting a release" section gains what cutting 0.9.0 depended on but no file recorded:

• the tag is signed and annotated with the message semvertag X.Y.Z (the command now says so);
• minor bump when the release carries a feat: or any behaviour change, patch otherwise;
• the Release body is generated from PR titles, so an upgrade-affecting change gets a ## Upgrading section via gh release edit.

Noticed, not changed here: under conventional-commits, a commit with a valid but non-bumping type (ci:, docs:) reports status no_conforming_commit with the reason "No conforming Conventional Commits type found in commit message." This PR's own merge will show that. The status token is frozen wire contract, but the reason text could distinguish "conforming, no bump" from "not conforming".

Verified with just lint-ci and just docs-build; the workflow YAML parses.