chore: prepare githits 0.9.3 and MCP 0.9.2 releases

[jlitola]

Summary

• finalize patch changelog sections for githits 0.9.3 and @githits/mcp 0.9.2
• keep CodeDiff unpromoted and absent from the served MCP tool set
• expose CodeDiff through a separate additive client capability so existing CodeNavigationService implementations remain source-compatible
• align root registry and generated plugin versions

Validation

• 2,903 tests passed
• typecheck, lint, formatting, build, and public package validation passed
• source and built CLI/MCP smoke suites passed, including authenticated live CodeDiff and CLI/MCP parity
• plugin generation and stale-asset checks passed
• MCP publisher v1.7.9 registry validation passed
• public registry domain proof, OAuth metadata, and unauthenticated challenge checks passed
• targeted standalone-site search agent evals passed for Claude and Codex through both MCP and skills surfaces

Merging this PR lets the successful Main workflow publish both npm artifacts, the root MCP registry entry, tags, and GitHub releases.