Fix CI: migrate from npm to pnpm for workspace:* protocol support

[Copilot]

CI was failing with npm error Unsupported URL Type "workspace:": workspace:* because the repository uses pnpm-style workspace protocol while the workflow used npm.

Package manager migration

• Add pnpm-workspace.yaml defining workspace structure
• Generate pnpm-lock.yaml, remove package-lock.json
• Update CI workflow: enable Corepack, cache pnpm, use pnpm install --frozen-lockfile

TypeScript fixes

• Remove unused imports and parameters (prefix with _ where semantically required)
• Add nullish coalescing for optional classification.confidence
• Filter undefined from labels array with type predicate
• Wrap boolean expression with Boolean() for strict typing

// Before: type error on possibly undefined
if (classification.confidence > 0.7) { ... }

// After: nullish coalescing with default
if ((classification.confidence ?? 0) > 0.7) { ... }

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[kayodebristol]

Draft PR noted in merge sweep. To get this ready to merge: (1) add/confirm a clear description of intent + testing notes, (2) ensure CI is green, (3) request review from required code owners (if any), (4) mark Ready for review once safe. If you want, I can help triage any failing checks.