feat(memory): SQLite database setup with migrations (#90)

[radoxtech]

Closes #90

Summary

Implements DC-MEM-001: SQLite database setup with migrations for the @diricode/memory package.

Changes

• Database client (src/db/client.ts): Singleton SQLite client using better-sqlite3 with WAL mode and foreign keys enabled
• Path resolution (src/db/path.ts): CWD-derived DB path resolution to .dc/memory.db with automatic directory creation
• Schema versioning (src/db/schema/version.ts): schema_versions table for tracking applied migrations
• Migration runner (src/db/migrations/runner.ts): Forward-only migration executor with transaction support
• Initial schema (src/db/migrations/001_initial_schema.ts): Creates observations and timeline_events tables

Epic Context

Part of Epic #11 — Memory and Project State Backbone [MVP-1]

Verification

• pnpm typecheck passes with zero errors
• pnpm build succeeds
• Functional tests verify singleton pattern, schema versioning, and table creation