parity(#211): revalidate cached MCP indexes on query (evict stale local-path caches)

[amondnet]

Summary

Upstream semble #211 (41c36f7) added on-query revalidation of cached MCP indexes for local paths. csp's MCP session cache does not revalidate on query, so a changed local repo can return stale results.

Upstream behavior (src/semble/mcp.py)

• _IndexCache._evict_if_stale(source, cache_key) runs on each query for local paths (git URLs skipped) and evicts the cached entry when the on-disk cache no longer matches the files.
• Gated by a cooldown: _revalidate_after[cache_key] = finished + (finished - start) * _MIN_REVALIDATE_FACTOR (_MIN_REVALIDATE_FACTOR = 3) so slow-to-build repos aren't rechecked too often.
• Uses semble.cache.get_validated_cache.

csp status

• crates/csp/src/mcp.rs has an LRU session cache + manual evict(), but no on-query staleness check / revalidate cooldown. A local path indexed once is served from the session cache even after its files change.

Proposed work

• Port _evict_if_stale + revalidation cooldown to mcp.rs (local paths only; skip git URLs).
• Reuse the existing manifest validation (content hash / chunk_size) as the staleness signal.

Refs

• Upstream: fix: Validate cached MCP indexes on query MinishLab/semble#211 (41c36f7)
• Sync baseline check: eacbe43..949559b, 2026-07-11