Warn MCP clients when a project index predates the current extraction version

[cbeaulieu-gt]

Summary

Surface a clear MCP warning when the selected project's index was built with an older extraction version than the running CodeGraph engine.

Context

getIndexBuildInfo() and isIndexStale() already provide a local compatibility check, and codegraph status reports when a full rebuild is recommended. MCP clients do not currently receive that warning through the default tool surface.

This is a smaller manual guardrail related to #1798, which proposes automatic background rebuilding. It does not implement automatic rebuilding. It also differs from #1243/#1253, which detects a newer published CodeGraph release rather than a stale project index.

Related work: #1842 and #1851.

Acceptance Criteria

• The first successful MCP tool response for a stale project warns that the index predates the running extraction engine.
• The warning appears once per project per MCP session.
• Explicit projectPath calls independently warn for each stale project.
• The message includes available build-version details and instructs the user to run codegraph index.
• codegraph_status always reports build-version details and whether reindexing is recommended.
• Current and uninitialized indexes do not produce the warning.
• MCP initialization remains nonblocking and preserves the existing fast-handshake contract.
• Detection is entirely local: no network request and no automatic reindex.
• Direct, proxy/daemon, and multi-project behavior have regression coverage.
• User-facing documentation or changelog text describes the warning.

Out of Scope

• Automatic rebuilding proposed by Automatically rebuild stale indexes in the background after engine upgrades #1798.
• Release-availability checks from codegraph upgrade should run automatically on MCP server start (or offer a check) #1243/feat(mcp): surface when a newer CodeGraph release exists (non-blocking update check) #1253.
• File-change and watcher staleness handled elsewhere.
• Automatically running codegraph index.

🤖 Generated by Codex on behalf of @cbeaulieu-gt