feat(skills): vendor writing-for-agents skill

[Quidge]

Intent

Vendor mattpocock's writing-for-agents skill into .agents/skills/writing-for-agents/ as the first skill brought in via the just-landed rebasing-adapted-skill machinery (PR #17). VERBATIM: SKILL.md, SKILL-MECHANICS.md, and agents/openai.yaml are copied byte-for-byte from upstream with zero edits (git blob hashes match upstream's pinned blobs); no reformatting or improvements. Vendored skills deliberately OMIT metadata.internal: true (that flag is only for firstmate's own skills); the presence of ADAPTATION.md is the orthogonal upstream-tracked signal, so its absence of that flag is intentional, not an oversight. ADAPTATION.md was scaffolded via the machinery's 'template' path and pins the attribution to the current 40-char commit SHA 4aaccb58d40559d7e3c59a029b2290ae5ba538de as a GitHub tree URL, with '## Deviations' declaring the single line '- no current deviations'. Provenance lives only in ADAPTATION.md, never in the vendored SKILL.md.

This PR also folds in a captain-approved refinement to the rebasing-adapted-skill machinery (skill-adaptation.py), authorized specifically to resolve a conflict discovered while using it: a byte-for-byte vendor has zero real differences from base, so the required human-readable '- no current deviations' line would otherwise be flagged by audit as a STALE DEVIATION. The fix makes a '## Deviations' section whose sole bullet is the literal '- no current deviations' a recognized sentinel meaning zero deviations (equivalent to an empty section), so a verbatim vendor audits clean while keeping the readable line; it collapses ONLY when it is the section's sole bullet. The convention is documented in the audit --help epilog, echoed in audit output, and in rebasing-adapted-skill/SKILL.md, and covered by three new offline tests in tests/fm-skill-adaptation.test.sh (clean sentinel; drift-under-sentinel is still undeclared drift; sentinel collapses only as sole bullet). This is a deliberate, scoped machinery change, not accidental scope creep. validate-skill-dir and audit both pass clean on the vendored dir; fm-lint.sh (shellcheck 0.11.0) is clean.

What Changed

• Vendor the upstream writing-for-agents skill with pinned provenance and register it for agent-runtime use.
• Teach adapted-skill audits to treat a sole - no current deviations bullet as a zero-deviation sentinel while still detecting undeclared drift.
• Document and add offline test coverage for sentinel auditing behavior and near-miss cases.

Pipeline

Updates from git push no-mistakes

✅ **intent** - passed

✅ **Rebase** - passed

✅ **Review** - completed

✅ **Test** - passed

✅ **Document** - passed

✅ **Lint** - passed

✅ **Push** - passed