fix: pack salience planner, config preflight, vector validation, 2 new proof scenarios

[hemang-doshi]

Summary

Implements remaining ProjectMind hardening phases: pack salience planner, config-driven preflight enforcement, vector read-only validation, and 2 additional causal proof scenarios.

Changes

Pack Salience Planner (Phase D)

• src/formatters/pack-planner.ts: planPackMemories() selects by salience before rendering
• Mandatory always included: pinned, priority>0, blocking command policies, high-severity fragile/failed/known_fix/agent_mistake
• Optional sorted by retrieval score; surplus becomes omitted summary
• Wired into formatPackMarkdown() replacing line-by-line truncation

Preflight Config Exit Codes (Phase F)

• preflight --enforce now uses config.preflight.enforce_warn_exit_code and enforce_block_exit_code
• Was hardcoded to 1/2 previously

Vector Read-Only Validation (Phase G)

• searchVectorIndexReadOnly() validates projectId, provider name, and dimensions
• Mismatches return empty results

ProjectMind Scenarios 5-6 (Phase H)

• architecture-decision: proves architecture notes surface for DB-related tasks
• supersession-visibility: proves old hidden after supersede, replacement hidden after quarantine
• 6/6 scenarios pass in isolated temp workspaces

Verification

pnpm typecheck              → zero errors
pnpm test                   → 32 files, 184 tests
pnpm build                  → clean
pnpm cli eval projectmind   → 6/6 pass
pnpm cli eval --json        → 5/5
pnpm cli eval live --json   → 8/8
pnpm cli benchmark run --all → 4/4