get_code_context refetches from GitHub on every call — add a cache

[GoodbyePlanet]

Impact: Medium (performance)

`server/tools/search.py:198-202` calls `fetch_file_content`, which opens a fresh HTTP client (`server/indexer/github_source.py:386`) on every invocation. The same file requested multiple times in a session hits the GitHub API each time.

Recommended change

The index already stores the blob SHA per file — add an LRU/TTL cache keyed on the blob SHA so repeated context fetches are served locally.