storage: SqliteAdapter (Phase 2 of #139)

[willwashburn]

Phase 2 of #139. Depends on Phase 1 (StorageAdapter seam) being merged.

Goal

Implement SqliteAdapter — a single-file SQLite database that replaces the JSONL ledger + per-session content sidecars + dedup-hash files for users who opt in via RELAYBURN_STORAGE=sqlite.

Uses Node 22's built-in node:sqlite (already used by archive.ts). DB path defaults to ~/.relayburn/burn.sqlite, configurable via RELAYBURN_SQLITE_PATH.

Schema

Mirrors the existing archive.ts table layout (sessions, turns, tool_calls, tool_result_events, compactions) and extends it with the record types currently held in JSONL only: relationships, user_turns, stamps, content. Dedup hashes from index-sidecar.ts (turnIdHash, relationshipIdHash, toolResultEventIdHash, userTurnIdHash, turnContentFingerprint) become PRIMARY KEY / UNIQUE columns so INSERT … ON CONFLICT DO NOTHING collapses duplicates natively — no separate .idx sidecar.

Migration files live under packages/ledger/src/adapters/migrations/ (shared with the Postgres adapter via small dialect-templating, no ORM).

Implementation notes

• WAL mode (PRAGMA journal_mode=WAL) for concurrent readers.
• withLock(name, fn) uses a locks table + BEGIN IMMEDIATE so multiple processes (e.g. ingest + MCP query) on the same file serialize correctly.
• Streaming query* paths use prepared statements with cursor iteration; no full-result materialization for large datasets.
• pruneContent becomes a parameterized DELETE FROM content WHERE ….

Files

• New: packages/ledger/src/adapters/sqlite-adapter.ts, packages/ledger/src/adapters/migrations/001_initial.sql, dialect-templating helper.
• New: packages/ledger/src/adapters/adapter-suite.test.ts — parameterized contract tests (append/dedup/query/lock semantics) that run against both file and sqlite.
• Modified: packages/ledger/src/adapters/factory.ts — wire up sqlite branch.

Verification

• The new parameterized adapter test suite passes against both File and Sqlite (same scenarios, same expectations).
• Manual: RELAYBURN_STORAGE=sqlite relayburn ingest against the same fixtures used in Phase 1 → relayburn analyze --json produces metrics matching the file-adapter run within rounding.
• Concurrent-writer smoke test: two relayburn ingest processes against overlapping inputs into the same SQLite file produce no duplicates and no SQLITE_BUSY errors.