fix: Exit the MCP server immediately on stdin EOF or SIGTERM

[Pringled]

MCP hosts stop the server by closing stdin or sending SIGTERM. Closing stdin during an index build or model load left the process running until that work finished, and SIGTERM exited with code 143, which Go-based hosts like Antigravity report as an error. The server now exits immediately with code 0 in both cases. This resolves #280

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
src/semble/cache.py	100.00% <100.00%> (ø)
src/semble/cli.py	100.00% <100.00%> (ø)

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[greptile-apps]

Confidence Score: 5/5

[High risk] Changes how the server shuts down and handles signals.

The PR appears safe to merge; no new actionable issue was identified.

_{Reviews (2) · Last reviewed commit: "Register SIGTERM handler earlier and tre..."}