[CI] Enforce conventional commits (convco) + pin release tooling (mise)

[justin13888]

Description

First PR in the release-automation stack for #18. Establishes conventional-commit enforcement everywhere and centralizes the release/commit toolchain in mise, so version syncing and CHANGELOG generation (later PRs) have a machine-readable history to build on.

What's added

• mise.toml (root) — pins the repo-wide release toolchain so local and CI install identical versions: just, lefthook, and convco (via the ubi: backend; it isn't in the mise registry). The Apple client's capsule-swift/mise.toml layers on top.
• .versionrc — convco config (auto-discovered). preMajor: true for 0.x semantics, plus changelog section mapping for the commit types already in use. Drives convco check/version/changelog.
• justfile — two recipes (justfile stays the SSoT for hooks + CI):
  • commit-check <msg_file> — validates a single in-progress message (convco check --from-stdin --strip); skips tooling-authored merge/revert/fixup/squash subjects.
  • check-commits [base] — validates a range (convco check --first-parent --ignore-reverts <base>..HEAD).
• lefthook.yml — new commit-msg hook (just commit-check) and a priority-0 pre-push range check (just check-commits).
• ci.yml — new commit-lint job (PRs only) that runs convco check over the PR's commits via mise-action, wired into the aggregated required gate (skipped on push → counts as pass).

Setup for contributors: run mise install at the repo root (installs convco/just/lefthook) — the commit-msg hook needs convco on PATH. Noted in the mise.toml header.

Verification (local)

• mise install → convco 0.6.4.
• commit-msg hook exercised by this PR's own commit: conventional message passes; a non-conventional message is rejected; Merge … subjects are skipped.
• just check-commits master → "no errors in 1 commit".
• pre-push ran check-commits on push (see branch).
• lefthook validate → All good; ci.yml/.versionrc/mise.toml parse.

Related Issues

Part of #18 (stack 1/3).

Contributor Checklist

• I have signed the Contributor License Agreement.
• My code follows the project's style guidelines according to CONTRIBUTING.md.
• Tests pass (commit-msg/pre-push/range checks verified locally)
• No sensitive info / secrets
• Docs updated if needed (mise.toml header documents setup)

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