feat(mcp): index_file reads from disk if content omitted

Forcing the agent to round-trip the entire file body through the tool
call was expensive on large files and discouraged calling the tool at
all after edits. Now `content` is optional — if absent, the server
reads filePath itself.

Agents that already have the bytes in working memory can still pass
content explicitly; that path is unchanged. The on-disk fallback is
purely additive.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>