feat(github): support in-place review thread replies

[CompleteDotTech]

Summary

• add a dedicated review-comment reply API route
• add in-place reply composer and accessible feedback to review-thread cards
• require an explicit numeric top-level review comment target for reply action markers
• expose reply metadata from the review-thread API and wire focused coverage

Bead

• cave-a2r8

Verification

• exact runner commands for reply route, API contracts, card wiring, marker directive, and GitHub blocks
• pnpm lint
• pnpm typecheck
• pnpm check:tests-wired
• pnpm check:conflict-markers
• git diff --check

Rebased onto current origin/main before publication.