fix: honor setting PATH remediations and register native MCP

[SebTardif]

Summary

Two live-red product bugs from MPI cycle 6.

When patchloom.path points at a missing binary, the status bar still
offered Install/Reinstall. Managed install succeeded, resolution still
used the broken setting, and the user looped. Not-ready remediations
now follow the same source rule as upgrades: Settings for
patchloom.path, Releases for PATH, managed install only when that is
the winning source.

Native MCP registration used API names that never existed
(registerMCPServerDefinitionProvider). VS Code 1.100+ never saw a
provider. The extension now registers registerMcpServerDefinitionProvider
with McpStdioServerDefinition, declares
contributes.mcpServerDefinitionProviders, and fires
onDidChangeMcpServerDefinitions after install/settings/trust.

Verification

• Unit tests for not-ready setting/PATH remediations
• Descriptor + contribution-point tests for native MCP
• npm run check

Checklist

• All commits in this pull request are signed off with git commit -s
• I ran npm run check and it passes
• I updated docs if user-facing behavior changed
• I am contributing this work under the repository license (MIT)