Add a new SDLC delegation recipe to the skill

[yuting0624]

skills/antigravity/SKILL.md has a routing table of when/how to delegate to agy.

Task: add one concrete, useful recipe (e.g. "generate tests for a module", "bulk-migrate imports", "summarize a large changelog") with the exact agy-delegate.sh invocation and the verification step Claude should run.

Acceptance: recipe follows the existing format and includes a verify gate. Keep it honest — only capabilities the code actually has.

[yuting0624]

One example of what this issue is asking for just landed in v0.16.0: the Internal fan-out recipe (skill section "Internal fan-out recipe (agy spawns its own subagents)") — one delegation, agy fans out internally to role-assigned self subagents, and each leaves a readable transcript.jsonl you can audit with the new agy-trace.

Keeping this issue open — the skill's recipe section can absolutely take more. Ideas that would make great contributions:

• Docs sweep: fan out over a large docs/ tree, one subagent per section, merge into a single digest
• Dependency upgrade: agy walks the changelog/breaking-changes of a major version bump and proposes the migration plan; Claude verifies against the real code
• Test-gap analysis: agy maps existing tests vs. public API surface and reports uncovered paths (read-only, cheap, perfect for --digest)
• PR description writer: agy drafts the description from the diff; Claude fact-checks claims against the actual changes

The pattern for any recipe: a concrete agy-delegate invocation, when to use it (and when NOT to — break-even), and how Claude verifies the result. See the existing recipes in skills/antigravity/SKILL.md for the shape.