fix(mcp): make non-trace explores side-effect free

[pratik-saptarshi]

Problem

run_fastcontext always appends --traj, so the nominally non-trace fastcontext_explore path still creates a trajectory file in temp storage. In src/fastcontext_mcp/runtime.py:206-217, the non-trace branch uses tempfile.gettempdir() rather than omitting trajectory capture entirely.

Evidence

• src/fastcontext_mcp/runtime.py:206-217 always extends the subprocess command with --traj.
• README.md:172-177 says trajectories are written only when requested, which does not match the current default path.
• tests/test_server.py:181-197 currently asserts that the non-trace path still receives a temp trajectory path.

Impact

This is an operational and privacy consistency issue. It adds hidden side effects to the non-trace API, creates unnecessary temp files, and contradicts the documented behavior that trajectories are only written when requested.

Recommended remediation

• Make trajectory emission opt-in only.
• fastcontext_explore should omit --traj entirely.
• Keep trajectory capture behind the explicit trace path or an explicit trace flag.
• If always-on traceability is intentional, update the docs to state that clearly and explicitly.

Regression tests to add or adjust

• Update the non-trace test to assert that --traj is not passed and no file is created.
• Add a trace-specific test that fastcontext_explore_with_trace still writes a trajectory file when explicitly requested.
• Update the README to match the final behavior.