Remove scoring engine and close superseded issues

[rigomart]

Part of #82

Depends on: #88 + future dimension and verdict issues (TBD)

Summary

Once the three-layer system (verdict + dimensions + red flags) is fully in place and no code paths reference the scoring system, remove the scoring engine from the codebase. This is the final cleanup step.

What gets removed

• Scoring engine — src/core/scoring/engine.ts (main calculation), quality.ts (weighted signal scoring), freshness.ts (freshness multiplier and hard caps), profiles.ts (score-to-category mapping and thresholds)
• Maintenance scoring API — src/core/maintenance/maintenance.ts and maintenance-config.ts (public scoring interface and category config)
• Score-related UI components — score.tsx, category-info-popover.tsx, confidence-indicator.tsx
• Score-related types — scoring types that are no longer referenced anywhere
• Tests — scoring engine tests, maintenance score tests, any test fixtures that only exist to test scoring

What gets kept

• Activity tier detection (src/core/scoring/signals.ts) — still used for contextualizing red flags
• All data collection code — GitHub API queries, metrics extraction, Convex storage
• Domain types for metrics snapshots — the data model is unchanged

Superseded issues

Close #61 ("idea: incorporate commit activity trend into score") and #70 ("feat: integrate commit activity into scoring engine and confidence penalties") as superseded — the scoring engine they aimed to improve no longer exists.

Acceptance Criteria

• All scoring engine code is removed (engine, quality, freshness, profiles, maintenance scoring)
• Activity tier detection logic (signals.ts) is preserved
• No dead code or orphaned imports remain referencing the scoring system
• All scoring-related tests are removed
• The build passes and type-checking succeeds with no scoring references
• Issues idea(scoring): incorporate commit activity trend into score #61 and feat(scoring): integrate commit activity into scoring engine and confidence penalties #70 are closed as superseded