feat(bot): add GitHub adapter

[RSO]

Summary

• Adds the Chat SDK GitHub adapter to the shared Kilo Bot infrastructure so GitHub issue and PR comment mentions route through the same bot flow as Slack.
• Preserves existing GitHub App webhook behavior for PR code reviews while forwarding GitHub comment events to the bot adapter.
• Adds GitHub platform identity handling and tests for GitHub webhook routing.

Verification

N/A (no manual verification performed).

Visual Changes

N/A

Reviewer Notes

• GitHub comment webhooks are now double-verified: once by the existing GitHub webhook handler and once by the Chat SDK adapter using a cloned request body.
• PR code review handling remains on the existing pull_request webhook path.