Add --review=code-reviewer option to /ship as alternative to gate2 cascade

[JFK]

Summary

Add a --review=code-reviewer flag to /gh-issue-driven:ship that skips the current gate2 cascade (audit + cso + qa-lead + cto parallel skills) and instead invokes the feature-dev:code-reviewer Agent for review.

Rationale: the feature-dev:code-reviewer agent has its own context window (can freely read changed files via tools, not just the prompt-embedded diff), uses confidence-based filtering to surface only high-priority issues, and covers security/bug/quality/conventions in a single pass. This offers a faster, more code-quality-focused alternative to the governance-lensed gate2.

Context

Priority: medium
Suggested labels: enhancement
Suggested milestone: (none)

Acceptance Criteria

• New flag --review=<target> added to /gh-issue-driven:ship argument parser.
  • Initial supported target: code-reviewer (maps to feature-dev:code-reviewer agent).
  • Reserved syntax for future targets (e.g. --review=code-review → code-review:code-review skill).
• When --review=code-reviewer is set:
  • Gate2 cascade (audit + cso + qa-lead + cto) is skipped entirely (same skip mechanism as auto-skip).
  • The feature-dev:code-reviewer agent is invoked via the Agent tool with the branch diff and PR context as input.
  • Agent output is parsed into pass | fail per ship.md's existing gate2 verdict contract.
• Verdict mapping: agent output is fail if it contains high-priority markers (must fix, blocker, critical, high-priority, etc.); otherwise pass. Exact marker list finalized during implementation, in the same heuristic-fallback style as start.md step 11.
• HITL behavior: matches current gate2 — green still requires HITL confirmation per the feedback_gate2_green_hitl.md memory.
• Interaction with auto-skip: orthogonal. If both flags are set, auto-skip wins (stricter omission — docs-only means "no review needed").
• If the feature-dev:code-reviewer agent is unavailable, the command must degrade gracefully — log a warning, fall back to the default gate2 cascade. No silent failure.
• /gh-issue-driven:doctor updated to report whether feature-dev:code-reviewer is available.
• README updates (EN + JA) documenting the new flag, its trade-offs vs gate2, and the orthogonality with auto-skip.

Design notes (non-blocking, for implementer reference)

• Precedent: auto-skip from PR feat(start,ship): token-efficient flags for cascade gating (#64) #65 already implements a gate2 short-circuit with a synthetic ## Verdict: pass and sentinel gate2.reviewer="diff-scope-skip". The code-reviewer path should mirror this — synthetic verdict + gate2.reviewer="code-reviewer-agent".
• Flag-naming convention (per session-2026-05-19 PR feat(start,ship): token-efficient flags for cascade gating (#64) #65 discussion): bare-word for heuristic flags, --prefix for skill/tool injection. --review= falls under the latter and leaves room for future targets.
• Out of scope (separate issues if pursued): auto-routing between gate2 / code-reviewer based on diff characteristics; supporting code-review:code-review skill as a target; promoting code-reviewer to default.