Summary

Enable the read-only POSIX MCP surface by default in v0.24 by changing enable_posix_tools from false to true.

This is the remaining product decision from #1398. It also finishes the discoverability work left on #1415 by documenting the project-as-mount namespace in the MCP server instructions.

Evidence

The completed evaluations favor the POSIX surface:

• final internal A/B: 12/12 tasks, 55% fewer tokens than the rich surface
• xAFS: 36/45 correct at 124K tokens per correct answer, versus 16/45 at 466K for rich
• Moby Dick qualitative run completed against a 206-note graph

The benchmark curation experiment is complete and is not part of this implementation.

Scope

• Change the default of enable_posix_tools to true.
• Preserve the configuration escape hatch so users can explicitly disable the tools.
• Add concise MCP server guidance explaining project-qualified paths and projects as mount points.
• Keep all existing rich MCP tools available.

Acceptance criteria

• A default configuration registers cat, grep, ls, find, tail, and man.
• Setting enable_posix_tools=false hides the POSIX tools.
• First-connect server instructions explain how project-qualified paths route.
• Clear integration tests cover the default-enabled and explicit-disable scenarios through the real MCP registration path.
• POSIX tool surface and memory evals (SPEC-47 A/B, BEAM, xAFS) #1398 and Project-qualified paths: projects as mount points in the tool namespace #1415 can close when the implementation lands.