fix: support Antigravity 2 migration paths

[kaitranntt]

Closes #889

Summary

• Update Antigravity migration targets for the 2.0 layout: agents merge into .agents/agents.md, project skills install to .agents/skills/<name>/SKILL.md, rules install to .agents/rules, workflows install to .agents/workflows, and global skills install to ~/.gemini/config/skills.
• Rewrite migrated markdown references so Claude .claude/agents/*, .claude/skills/*, .claude/commands/*, .claude/rules/*, and hook references point at the Antigravity 2.0 equivalents.
• Add reconcile/dry-run visibility and cleanup guard coverage for migrated skill directories so reruns show and converge on the same paths users actually see in Antigravity.

Validation

• bun test src/commands/migrate/__tests__/migrate-command.integration.test.ts src/commands/migrate/__tests__/skill-directory-installer.test.ts src/commands/skills/__tests__/skills-installer.test.ts src/commands/portable/__tests__/portable-installer.test.ts __tests__/commands/portable/converters.test.ts src/commands/portable/__tests__/md-strip.test.ts
• bun test (5031 files/test cases run: 4972 pass, 59 skip, 0 fail)
• bun run typecheck
• bun run lint
• bun run build
• bun run help:check-parity
• Pre-push local CI parity, package verification, and UI vitests
• Local isolated smoke: ck migrate --agent antigravity --yes produced .agents/agents.md, .agents/skills/cook/SKILL.md, .agents/rules/style.md, and .agents/workflows/docs-release.md with stale .claude/* refs removed from managed markdown.

Docs Impact

• Regenerated docs/cli-reference.md and cli-manifest.json for the new migrate help text.
• Follow-up docs PR: docs: document Antigravity 2 migrate paths claudekit/claudekit-docs#174.

[github-actions]

🎉 This PR is included in version 4.5.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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