feat(ship): add external code review command hook to ship workflow

Adds workflow.code_review_command config key that allows solo devs to
plug external AI review tools into the ship flow. When configured, the
ship workflow generates a diff, builds a review prompt with stats and
phase context, pipes it to the command via stdin, and parses JSON output
with verdict/confidence/issues. Handles timeout (120s) and failures
gracefully by falling through to the existing manual review flow.

Closes #1876

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

get-shit-done/templates/config.json

@@ -11,7 +11,8 @@
     "security_asvs_level": 1,
     "security_block_on": "high",
     "discuss_mode": "discuss",
-    "research_before_questions": false
+    "research_before_questions": false,
+    "code_review_command": null
   },
   "planning": {
     "commit_docs": true,