ci(mypy): unblock strict gate from numpy PEP 695 stub

[lyonzin]

Summary

Unblocks Pillar 7 — mypy strict (gradual) which has been failing on every push to master since the numpy stub started using PEP 695 type statements.

Root cause

Recent numpy ships __init__.pyi with PEP 695 syntax:

```
/opt/.../numpy/init.pyi:737: error: Type statement is only supported in
Python 3.12 and greater [syntax]
```

Our mypy config pins python_version = "3.11" (project still supports 3.11 at runtime), so mypy rejects the stub. None of the three checked modules import numpy directly — it enters through:

```
mcp_server/preflight.py
-> mcp_server.config
-> chromadb / fastembed
-> numpy <-- explosion happens here
```

Fix

Add a single [[tool.mypy.overrides]] block scoping the stub-skip strictly to numpy. ignore_missing_imports = true doesn't help here because numpy has stubs — mypy loads them and chokes on parse. follow_imports = "skip" tells mypy not to parse the numpy module even when transitively imported.

Verified locally

```
$ mypy mcp_server/instance_lock.py mcp_server/preflight.py scripts/
Success: no issues found in 7 source files
```

Why not the alternatives

• Bump python_version = "3.12": changes the assumed runtime target for the entire codebase, even though the package still supports 3.11. Risk of masking real 3.11-only issues. Out of scope.
• Pin a specific numpy/mypy version: brittle and chases the same problem forward; future stub releases will break us again.
• Wait for upstream mypy fix: no public timeline, master stays red in the interim.

7 Pillars

• No production code touched; config-only
• No test changes; backwards-compat preserved
• No new dependencies
• mypy clean locally on the gradual-allowlist
• CI green on all 9-cell matrix + 7 pillars (pending)