feat(integration): add Antigravity CLI (agy) integration support

[bxshi]

Summary

Adds support for Antigravity CLI (agy) as an official agent integration target in Herdr.

• Adds IntegrationTarget::Agy schema and registry setup.
• Bundles agy lifecycle state hooks for Bash and PowerShell (src/integration/assets/agy/).
• Supports session identity reporting (herdr:agy) to enable native session restore (agy --resume <session_id>) upon Herdr server restart.
• Extends CLI commands (herdr integration install/uninstall/status agy) and usage messages.
• Updates documentation (integrations.mdx).

Testing

• Verified cargo test and bun test suites.
• Verified cargo clippy --all-targets --locked -- -D warnings.
• Verified herdr integration status and herdr integration install agy end-to-end.

[kangal-bot]

Hi @bxshi, thanks for your interest in contributing!

Herdr automatically admits focused bug fixes from contributors who are not maintainers when the title uses fix: ... or fix(scope): ... and the patch changes no more than 20 files and 1,000 total added or deleted lines.

Contributors who are not maintainers may submit only focused bug fixes. If this pull request fixes a bug, rename it to use a conventional fix: ... or fix(scope): ... title, then tag a maintainer to review and reopen it.

Feature requests, behavior changes, and other proposals belong in GitHub Discussions and require maintainer approval before a pull request.

If this gate classified the pull request incorrectly, reply and tag a maintainer listed in .github/MAINTAINERS. A verified maintainer can reopen it; reopening by anyone else will be closed again automatically.

Patch size: 13 changed files, 465 changed lines.

See https://github.com/herdrdev/herdr/blob/master/CONTRIBUTING.md for the contribution policy.

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[bxshi]

NVM see opening PR for this #2087