feat: commit-convention-aware docs-sync commits

[heysanil]

What & why

The docs-sync commit's subject (docs: sync documentation with code changes) and PR title
were hardcoded strings, and the commit body had no line-length discipline. In repos that lint
their commit history (commitlint, gitlint, cocogitto, …) this can fail the calling repo's own commit
lint — e.g. @commitlint/config-conventional enforces body-max-line-length: 100 at error level,
and a repo whose type-enum omits docs or requires a scope rejects the hardcoded subject.

This makes the sync commit/PR detect the calling repo's commit convention and shape its subject +
body to match, with a tunable fallback body line width.

Design

Hybrid ownership — the shell keeps every safety guarantee; the auditor only shapes prose:

Concern	Owner	Guarantee
[skip docs-sentinel] loop-breaker	shell	Always appended, relocated from the subject to a footer trailer so it never trips a header-length/subject rule. The gate greps the whole message, so a footer marker is behaviorally identical for the loop guard.
Commit subject	agent proposes, shell validates	Auditor emits a Subject: line; shell uses it only if non-empty and ≤100 chars, else falls back to the static subject.
Body prose + wrap	agent (best-effort)	Prompt-directed; no mechanical shell reflow (would corrupt markdown/code/URLs).

Precedence for the wrap width: detected repo body-max-line-length > the commit-body-line-length
fallback (default 100). So linted repos get their real value; unlinted repos get sane plain-text
hygiene.

Changes

• engine/compose-message.sh — peel/validate an optional Subject: line off the auditor
  summary; emit it as the commit subject + new pr_title step output; move the skip marker to a
  footer trailer.
• engine/prompt-skeleton.md — new "Matching the repo's commit convention" section
  (glob-cheap detection of commitlint / commit templates / commitizen-cocogitto-gitlint) + the
  Subject: output contract.
• .github/workflows/audit.yml — new commit-body-line-length input (default 100), validated
  as a positive integer and substituted into the prompt in both audit jobs; sync-PR title: now
  uses steps.compose.outputs.pr_title.
• README.md / AGENT_SETUP.md — document the input + behavior.
• Design spec + implementation plan under docs/superpowers/.

Testing

Local run of the three CI gates — all clean:

• shellcheck engine/*.sh
• bats tests — 24/24 (new cases: subject extraction, footer-marker placement, pr_title output,
  empty/over-long subject fallback)
• actionlint

Plus an end-to-end run of compose-message.sh with a Subject: line, and a simulation of the
workflow's prompt substitution + input validation.

Integration caveat: the subject-detection path runs the real Claude Code CLI in CI, so it can
only be fully exercised on a live PR — the local suite covers the shell contract, not the model's
output.