feat(agents): add dispatch-implement wrapper skill for Claude Code (#210)

[jinyeow]

What

Adds ai-agents/skills/dispatch-implement/SKILL.md, a thin wrapper skill that dispatches
one subagent per ticket to run /implement unmodified, for Claude Code.

• Parses one or more #ticket references from the invocation.
• Judges file/scope overlap pairwise; dispatches in parallel only when every pair is
  confirmed non-overlapping, otherwise sequential (conservative by default).
• Picks a model per ticket under the existing implement-stage pin (Opus or Sonnet, never
  Fable/Haiku), honoring an explicit override where one is given.
• Dispatches via the Agent tool (subagent_type: general-purpose, so /implement's own
  /tdd//code-review//spec-review//codex-review calls still work), with
  isolation: "worktree" on parallel children to avoid a shared .git/index race.
• Integrates parallel children by cherry-picking each child's commits onto the invoking
  branch in ticket order, running fast checks once over the result, then cleaning up child
  worktrees and branches. Stops on a cherry-pick conflict rather than resolving it silently.
• Registers the new skill in ai-agents/SKILL-OWNERSHIP.md, Claude Code only for now;
  Codex CLI (feat(agents): dispatch-implement — Codex CLI dispatch #211) and Pi (feat(agents): dispatch-implement — Pi dispatch #212) dispatch are tracked separately and stop-and-report until
  wired.
• Parallel dispatch is gated off: every ticket runs sequentially regardless of the
  overlap judgment, until chore(agents): confirm dispatch-implement parallel dispatch after trial week #213 confirms the base flow against real usage and lifts the
  gate.

implement/SKILL.md itself is untouched (verified via diff), per
docs/adr/implement-stays-minimal-dispatch-in-wrapper-skill.md.

Why

Per #207 / .agents/specs/dispatch-implement.md: the user regularly hands off tickets by
typing "dispatch subagent(s) to /implement #123" by hand. This automates that habit
without folding dispatch logic into /implement itself, which the linked ADR rejects as
breaking its minimal upstream shape.

Verification

• git --no-pager diff main...HEAD -- ai-agents/skills/implement/SKILL.md returns empty,
  confirming /implement is byte-unchanged.
• Reviewed by a Fable subagent and Codex (cross-model second opinion); findings applied in
  a follow-up commit (cherry-pick range/abort mechanics, concrete overlap rules, parsing
  edge-case guards, explicit fast-checks-only tradeoff, child-branch cleanup).
• Not yet exercised against a real dispatch run. chore(agents): confirm dispatch-implement parallel dispatch after trial week #213 tracks the trial and lifting the
  sequential-only gate.

Closes #210.