Add implement skill to drive the per-ticket build loop

[bracyw]

What

Adopt Matt Pocock's implement skill: the conductor for building one ticket at a time. It branches per the repo convention, drives tdd at pre-agreed seams, runs checks as it goes, reviews with code-review, and commits via the Argos /commit convention. Does not push or open PRs.

Notes

• Reuses the existing /code-review rather than vendoring Matt's (avoids shadowing the built-in).
• Closes out the stage #668 - Branch-based planning pipeline and ai-workflow label #669 left manual: to-spec to to-tickets to implement per ticket.

Acceptance criteria

• .claude/skills/implement/SKILL.md added, Argos-adapted (commit via /commit, repo test commands, branch convention)
• Wired into the main-flow narrative in CLAUDE.md

Supersedes #682 — implement plus the {issue}-{kebab-title} branch-from-develop convention cover what a standalone /start-ticket skill would have. Part of the Matt-Pocock-skills sync (#683/#684/#689/#691).