feat(harness): PR-title CI lint, contexts/docs boundary, commit-body rule, and repo-root-relative refs

[keli-wen]

Summary

Finishes the harness work in #134: the last open acceptance criterion (the contexts/ vs docs/ boundary) plus the server-side floor for the commit convention, and it codifies the commit-body wrapping rule we agreed on.

• PR-title CI lint — .github/workflows/pr-title.yml lints the PR title (which becomes the squash-merge commit subject) as an English Conventional Commit, reusing scripts/hooks/commit_msg_check.py so the rule has a single source. The local commit-msg git hook only fires for contributors who ran pre-commit install, so it cannot bind external contributors; this workflow is the server-side floor that does. Making it a required status check is a one-time repository-settings toggle, separate from this PR.
• contexts/ vs docs/ boundary — contexts/README.md now states it explicitly under "Where Information Lives": contexts/ is internal navigation and design memory for coding agents and maintainers; docs/ is the user-facing catalog (docs/README.md) and usage guides (docs/library.md). This was the one unmet acceptance criterion in feat(harness): agent-facing context map + convention-enforcing git hooks #134.
• Commit-body wrap rule — commit.md (both skill copies) adds the convention: a commit body follows the same no-hard-wrap prose rule as GitHub bodies, so a squash-merge keeps a clean description. This PR's own commit body follows it.
• Doc — contexts/dev/harness-engineering.md records the PR-title lint in the enforcement table and replaces the earlier "planned next" note.
• Repo-root-relative cross-references — nested skill files (SKILL.md, pull-request.md, commit.md) referenced contexts pages with deep, fragile links like ../../../../contexts/dev/github-writing.md. A bare clickable relative link only resolves against the current file's directory, so a nested file needs that ../ chain to stay clickable; the sister repo llmquant-data-mono avoids it by using repo-root-relative backtick references (contexts/...) from nested files. This PR adopts the same convention: nested files use backtick repo-root references, while root-level files (AGENTS.md) and sibling contexts pages keep their clean clickable links. tests/test_contexts.py now asserts the reference in the form appropriate to each source location plus repo-root existence, instead of a specific deep relative path.

Related Issue

Closes #134.

Verification

• bash scripts/verify.sh passes: ruff format + ruff check + basedpyright + import-linter + pytest --cov (426 passed, 85.65% coverage). The two commit.md copies stay identical, and the edited contexts/ pages keep their required structure — both enforced by tests/test_contexts.py.
• The reused rule was smoke-tested against a sample PR title, and pr-title.yml was validated as YAML. The workflow itself runs on GitHub against a pull request, so its end-to-end behavior is exercised by this PR's own title check rather than locally.
• No live-network component smoke test applies: this PR adds a CI workflow, a documentation boundary, and a commit convention; it changes no public operation or source.

Checklist

• The title uses English Conventional Commit format: type(scope): summary.
• The related issue or design discussion is linked when applicable.
• bash scripts/verify.sh passes.
• Every applicable live-network component smoke test passes, or this PR states why none applies.
• Public behavior has focused tests, an example, and documentation where applicable.
• The PR is complete, small, and contains no unrelated changes.