release: v3.0.0 — VS Code extension + full editor integration

[corvid-agent]

Summary

• VS Code extension v1.0.0 — first-class editor integration with inline diagnostics, CodeLens quality scores, coverage/scoring webviews, status bar indicator, and validate-on-save
• CLI bumped to v3.0.0 — major version to reflect the extension as a new surface area
• Docs updated — README gains a full VS Code Extension section, CHANGELOG has detailed v3.0.0 notes, SCOPE.md updated to v3, GitHub Action refs bumped to @V3

What's in the extension

Feature	Description
Inline diagnostics	Errors/warnings from specsync check --json mapped to spec files
CodeLens scores	Quality scores (0–100 with letter grades) inline above specs
Coverage webview	Rich HTML report with file + LOC coverage
Scoring webview	Per-spec quality breakdown with suggestions
Status bar	Persistent pass/fail/error/syncing indicator
Validate-on-save	500ms debounced automatic validation
5 commands	Check, Coverage, Score, Generate, Init via Command Palette

Verification

• ✅ 74 integration tests passing
• ✅ Clippy clean (zero warnings)
• ✅ Extension typechecks clean (tsc --noEmit)
• ✅ Extension bundles successfully (17kb output)

Test plan

• Install extension in VS Code and verify activation on workspace with specsync.json
• Run each of the 5 commands from Command Palette
• Verify diagnostics appear on spec files after save
• Verify CodeLens scores display on .spec.md files
• Test coverage and scoring webview reports render correctly
• Verify status bar shows correct state after check runs
• Test with specsync.binaryPath pointed to a custom location

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