feat(session-persistence-sqlite): second backend validating the abstraction

Add a SQLite SessionPersistence backend (node:sqlite), a SECOND
implementation built to prove the abstract seam + the shared
runPersistenceContract suite are genuinely backend-agnostic. Each
SessionEvent maps 1:1 onto an events row (session_id, seq, type, time,
data); append is an INSERT inside a transaction asserting the
contiguous-seq contract; the mutable SessionSummary lives in the
sessions metadata row.

It satisfies the SAME contract semantics as the JSONL backend, expressed
over rows instead of file bytes:
- Lazy materialization: create() records intent in memory; no row until
  the first append (a never-appended session is absent from has()/list()
  via a materialized flag set inside the first append transaction).
- Crash-tail-on-load: load() returns events only through the last
  complete turn/end and deletes the uncommitted tail; a seq gap in the
  committed region makes the session unloadable.
- Transactional append: a mid-batch failure (a UNIQUE seq collision from
  a concurrent writer) rolls back entirely, keeping the cursor truthful.

Like the JSONL backend it is also the write-path plugin (session/event →
buffer → session/flush drain, onCreated seed/adopt/collision handling,
HMR seeding, dispose-to-quiescence). The package runs the shared
runPersistenceContract suite plus SQLite-specific tests (transaction
rollback, crash-tail cut, schema version, HMR adoption).

Docs flip every "SQLite is future/deferred" reference (ADR 0016,
architecture.md, the persistence module doc + README) to "implemented;
the contract holds both backends to identical semantics".

packages/session-persistence/src/index.ts

@@ -4,9 +4,12 @@
  * list, and update sessions — without saying HOW. Implementations subclass
  * {@link SessionPersistence} and register themselves as the
  * `sessionPersistence` service; `@deepseek-ai/dsh-session-persistence-jsonl`
- * (an append-only JSONL log per session) is the first. Future backends swap in
- * SQLite/WAL, an object store, or a remote service without touching the
- * consumers (the write-path plugin, the agent-loop resume seam).
+ * (an append-only JSONL log per session) is the first and
+ * `@deepseek-ai/dsh-session-persistence-sqlite` (`node:sqlite`, one row per
+ * event) is a second that validates the seam is backend-agnostic by passing
+ * the same `runPersistenceContract` suite. Further backends swap in an object
+ * store or a remote service without touching the consumers (the write-path
+ * plugin, the agent-loop resume seam).
  *
  * The persisted unit IS the existing {@link SessionEvent} — there is no
  * parallel "persisted message" type the log must be converted to and from