docs(mcp): add local code lookup with Sem

[rs545837]

Summary

Add an optional Sem stdio configuration alongside the existing MCP examples. Documents explicit checkout selection, tool discovery, removal, index writes, and process permissions. No defaults, runtime behavior, or dependencies change.

I maintain Sem.

Verification

• Checked the documented command shape against the MCP CLI argument definitions and existing examples.
• Sem 0.27.0 stdio initialization, tool discovery, and sem_find lookup passed against a disposable Git checkout; returned the target and caller.
• git diff --check passed.
• Rustcode itself was not executed. An offline workspace check was blocked by uncached crossterm; the contribution guide exempts docs-only changes from Rust build checks.

The example does not claim performance improvements or treat SEM_REPO as a sandbox.

[LHagfoss]

Thanks Rohan, nice one! Sem looks really cool, and I like that you spelled out the SEM_REPO caveat and that the validation tools can run commands. Merging it in, it'll go out in the next release. 🙌