feat(mcp): enable POSIX tools by default for v0.24

[phernandez]

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.

[claude]

Claude finished @phernandez's task in 36s —— View job

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Triage

Type: Enhancement — flips a config default (enable_posix_tools: false → true) plus documentation/tests, no new capability being invented.

Component: MCP (server config/tools, src/basic_memory/config_models.py, src/basic_memory/mcp/tools/posix_tools.py, src/basic_memory/mcp/server.py).

Priority: Medium — no breakage or data-loss risk; the escape hatch keeps this reversible, but it changes the default tool surface for every user, so it's worth landing deliberately rather than urgently.

Complexity: Medium — the code change itself (default flip) is small, but the issue also asks for new server-instruction copy on project-qualified paths and integration tests through the real MCP registration path, which is more than a one-line diff.

Verification: Confirmed current default is indeed False in config_models.py:618-619, so the premise is accurate. #1398 (open) is the parent epic this closes out; #1415 (closed) is the discoverability work being extended. No duplicate issue found via gh search issues.

This is a clear, well-specified enhancement ready for implementation — no clarifying questions needed.