Add contributing guide with pre-commit hook for skill sync

[sebastientaggart]

Adds a contributing guide documenting the pre-commit hook that enforces skill file sync, and syncs the currently out-of-date generated setup skill files.

Issue #115

[sebastientaggart]

Review Summary

Verdict: APPROVE

Findings

• [NOTE] The four adapter files (.agents/skills/setup/SKILL.md, .claude/commands/setup.md, .cursor/rules/setup.mdc, .gemini/skills/setup/SKILL.md) contain identical logical changes with updated sync hashes, confirming sync.sh was run correctly.

• [NOTE] The "Greenfield GitHub baseline guidance" section was removed as a standalone Phase 3 preamble but its content was preserved as a condensed inline note in Phase 2 (label check step), which keeps the guidance available without the multi-step interactive flow. This is a net simplification that retains the important information.

• [NOTE] The Standard profile previously specified DEFAULT_REVIEWERS and TICKET_LABEL_CREATION_ALLOWED should be skipped for the Standard profile in Phase 3 only "unless the user asks about them." The new table-based format drops that nuance and now lists them as applicable to Standard unconditionally, which is arguably more consistent and correct (they were already walked through for Standard in the old version's per-key sections).

• [NOTE] The pre-commit hook in docs/contributing.md uses ./sync.sh --dry-run 2>/dev/null which silences stderr. If sync.sh itself has an error (e.g., missing dependency), the hook would silently pass. This is minor since the hook is a developer convenience, not a CI gate, but worth being aware of.

No correctness, security, or convention issues found. The PR is a clean refactor that deduplicates the setup skill's verbose per-profile and per-value instructions into tables and shared patterns, and adds a contributing guide documenting the pre-commit hook.