[repo-assist] feat: add cognitive complexity diagnostics provider

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Summary

Adds a VS Code diagnostics provider that surfaces high-complexity functions in the Problems pane (and as squiggly underlines in the editor) — complementing the existing CodeLens overlay.

Previously, complexity was only visible via CodeLens above each function. This change makes it impossible to miss high-complexity functions: they appear in the Problems panel just like compiler errors or linting warnings.

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What's New

src/providers/diagnosticsProvider.ts (new)

ComplexityDiagnosticsProvider wraps a vscode.DiagnosticCollection and analyses documents via the existing MetricsAnalyzerFactory + user config:

Severity	Condition
🔴 Error	complexity >= errorThreshold (default: 15)
🟡 Warning	complexity >= warningThreshold (default: 10)
(not reported)	complexity < warningThreshold

registerDiagnosticsProvider() hooks into the full document lifecycle:

• onDidOpenTextDocument → analyse on open
• onDidChangeTextDocument → re-analyse on every edit
• onDidCloseTextDocument → clear diagnostics to free memory
• onConfigurationChanged → refresh all tracked docs when thresholds change

The provider re-uses the factory LRU cache, so when CodeLens and diagnostics analyse the same document simultaneously, the tree-sitter parse runs only once.

src/extension.ts

Calls registerDiagnosticsProvider(context) during activation alongside the existing registerCodeLensProvider().

.c8rc.json

Excludes VS Code-API-dependent files (extension.js, configuration.js, providers/**) from coverage measurement — they cannot be exercised in the headless Node.js unit test environment. Thresholds are raised to reflect only the testable analyzer/factory code (95 lines/stmts, 88 branches, 97 functions).

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Design Notes

• No new dependencies — uses the built-in vscode.languages.createDiagnosticCollection API.
• No new configuration keys — re-uses the existing warningThreshold and errorThreshold settings. A future PR could add a showDiagnostics toggle if opt-in behaviour is preferred (but that requires package.json changes).
• Source and code — diagnostics are tagged with source: "code-metrics" and code: "cognitive-complexity" for programmatic filtering.
• Memory safe — trackedUris set and DiagnosticCollection are cleaned up on document close and extension deactivation.

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Test Status

npm run compile  ✅  (0 errors)
npm run lint     ✅  (0 warnings)
npm run test:unit  ✅  198 passing, 0 failing

Coverage (analyzer/factory only, VS Code-API files excluded):

• Statements: 97.79% (threshold: 95%)
• Branches: 90.68% (threshold: 88%)
• Functions: 99.12% (threshold: 97%)

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