Agent Skills: code-implement

I-phase orchestration skill per Lock 10. Iterates over the locked plan's slices in dependency order. For each slice: invokes worktree-setup (first slice only or once per parallel plan); dispatches `codesmith-worker` subagent with slice scope (plan + slice boundary + verification commands + commit convention + worktree path); awaits PHASE_COMPLETE or NEEDS_DECOMPOSITION; on decomposition triggers mini-replan per Lock 19; on completion invokes `code-verify` for the I-per-slice gate. Owns the plan-as-living-doc's structural sections; worker owns the per-slice log subsections. Tracks decomposition strike count (abort after 3 consecutive on the same slice per Lock 17.2).