feat: prepare v1.5.0 trust and indexing foundation

[PatrickSys]

Summary

• Establish the v1.5.0 foundation for trust-aware editing: evidence-lock preflight, memory confidence decay, failure memory weighting, and git-derived memory extraction.
• Add manifest-based incremental indexing (file hash diff + incremental stats + stale chunk replacement) so refreshes can skip unchanged files.
• Strengthen storage/index resilience around LanceDB corruption handling and index lifecycle behavior.

What is included in this PR

• Preflight trust system: src/preflight/evidence-lock.ts, preflight integration in src/index.ts
• Memory reliability: confidence decay model + stale filtering in src/memory/store.ts, git memory extraction in src/memory/git-memory.ts
• Incremental indexing core: new manifest logic in src/core/manifest.ts, indexer integration in src/core/indexer.ts
• API/types plumbing: updates in src/types/index.ts, src/storage/types.ts, src/storage/lancedb.ts, constants
• Regression coverage: new tests for evidence lock, confidence decay, git memory patterns, manifest, and incremental indexing
• Post-review fix: invalid memory.date now evaluates as stale evidence (commit 5fa2040)

Validation

• Local: pnpm lint ✅
• Local: pnpm format:check ✅
• Local: pnpm test -- tests/confidence-decay.test.ts ✅

Notes on docs/changelog split

• Detailed release-note alignment (README + CHANGELOG wording) is intentionally handled in stacked PR docs: align release notes with shipped search and incremental behavior #23 (v1.5.0/docs-facelift) to keep this foundation PR focused on core behavior and testability.
• This PR still includes foundational docs touch-ups (README.md, MOTIVATION.md, AGENTS.md) but the final release messaging pass is in docs: align release notes with shipped search and incremental behavior #23.

Why this split

• Keep reviewable surface area manageable in a stacked release:
  1. foundation behavior and tests
  2. reliability hardening
  3. final docs/release wording
• Prevents mixing large implementation review with marketing/release copy edits in one PR.

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