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".

yarn.lock

@@ -706,6 +706,21 @@ __metadata:
   languageName: unknown
   linkType: soft
 
+"@deepseek-ai/dsh-session-persistence-sqlite@workspace:packages/session-persistence-sqlite":
+  version: 0.0.0-use.local
+  resolution: "@deepseek-ai/dsh-session-persistence-sqlite@workspace:packages/session-persistence-sqlite"
+  dependencies:
+    "@deepseek-ai/dsh-session": "npm:^0.0.1"
+    "@deepseek-ai/dsh-session-persistence": "npm:^0.0.1"
+    cordis: "npm:^4.0.0-rc.6"
+    schemastery: "npm:^3.18.0"
+  peerDependencies:
+    "@deepseek-ai/dsh-session": ^0.0.1
+    "@deepseek-ai/dsh-session-persistence": ^0.0.1
+    cordis: ^4.0.0-rc.6
+  languageName: unknown
+  linkType: soft
+
 "@deepseek-ai/dsh-session-persistence@npm:^0.0.1, @deepseek-ai/dsh-session-persistence@workspace:packages/session-persistence":
   version: 0.0.0-use.local
   resolution: "@deepseek-ai/dsh-session-persistence@workspace:packages/session-persistence"