chore(release-skill): resolve branch commit vs. commit-msg hook conflict

[brunopgalvao]

Summary

During the v0.17.0 release cut, Phase 4 step 2 of the release skill hit the repo's commit-msg hook:

• SKILL.md instructed git commit -m \"Release vX.Y.Z\" on the release branch.
• The hook (.cargo-husky/hooks/commit-msg) rejects that subject — it enforces conventional-commit prefixes and only exempts chore(release).

The fix exploits the fact that GitHub derives the squash-merge subject on `master` from the PR title, not the branch commit. So the skill now uses two different subjects:

• Branch commit: `chore(release): vX.Y.Z` — passes the hook, no `--no-verify` required.
• PR title: `Release vX.Y.Z` — becomes `Release vX.Y.Z (#N)` after squash-merge, which matches the downstream regex `^Release v[0-9]+\.[0-9]+\.[0-9]+ \(#[0-9]+\)$` used by `publish-release.yml`.

Changes

• `SKILL.md` Phase 4 step 2: branch commit now uses `chore(release): vX.Y.Z`; added explicit note that `Release vX.Y.Z` belongs on the PR title, not the branch.
• `COMMIT_CONVENTIONS.md`: split "Release commit" section into branch subject + PR title; added explicit "do not" entries against `--no-verify` and against putting `Release v{VERSION}` on the branch commit.

Test plan

• v0.17.0 release PR (Release v0.17.0 #284) used the new two-subject pattern; `commit-msg` hook passed without `--no-verify`.
• Next release cut follows the updated SKILL.md without re-discovering the hook conflict.
• On squash-merge of Release v0.17.0 #284 to master, the resulting commit subject matches `^Release v[0-9]+\.[0-9]+\.[0-9]+ \(#[0-9]+\)$` and `publish-release.yml` tags + publishes `v0.17.0`.