fix(harness): close Pi MCP bridge invariant gaps

[solomonneas]

Summary

• Reject Pi MCP catalogs whose enabled server names contain the reserved __ separator, preventing qualified-name collisions and call misrouting.
• Treat MCP result.isError: true responses as tool failures while preserving server, tool, and qualified-name identity.
• Add regression coverage for deterministic naming, stdio and HTTP catalog resolution, receipt-based uninstall, and Node built-in-only extension imports.

Root cause

The bridge split qualified names on __ without reserving that separator in server names, so two server/tool pairs could produce the same name. It also checked only JSON-RPC error responses and missed MCP result-level failures. The install projection now uses the same reserved-name validation as runtime discovery, so it cannot report an unusable catalog as ready.

Verification

• brigade work verify run --target . --command "pytest -q tests/test_pi_mcp_bridge.py" --capture brigade-work
• brigade work verify run --target . --command "./scripts/verify" --capture brigade-work

Fixes part of #438.

[coderabbitai]

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