feat(storage): make SQLite the default session metadata store

[likun666661]

Part 2 of #1370.

Depends on #1371. Please review/merge that foundation PR first; this is a stacked draft PR and will temporarily show the Part 1 commits until its dependency lands.

What changes

• Makes createSessionStore() use SQLite as the canonical store for all SessionHeader metadata.
• Atomically imports existing JSONL line-1 headers into a dedicated sessions.sqlite database on startup.
• Keeps JSONL as the transcript body store during the compatibility window; metadata updates no longer rewrite the legacy header.
• Adds deletion tombstones so a leftover legacy transcript cannot resurrect a removed session after a crash.
• Preserves session listing, recovery validation, connection-lock self-healing, archive/flag/read/title operations, and transcript previews.
• Closes the SQLite handle during desktop and CLI shutdown.
• Keeps session metadata in sessions.sqlite rather than runtime.sqlite, so this cutover does not implicitly change the existing opt-in RuntimeEvent rollout.

Compatibility boundary

Fresh sessions still receive a frozen legacy header as the JSONL first record. SQLite is authoritative immediately; Part 3 will replace that record with an explicit transcript marker and add export/downgrade/backup coverage.

Validation

• npm test --workspace @maka/storage: 435 passed, 1 skipped
• npm run build:test: passed across all workspaces and desktop bundles
• npx biome lint on all 11 changed files: passed
• git diff --check: passed
• npm run test:dist: storage and other workspaces passed; @maka/runtime has 2 unrelated macOS sandbox/toolchain failures (hard-coded /usr/local executable-root expectation and Homebrew rg blocked from loading PCRE2). Runtime result: 2377 passed, 7 skipped, 2 failed.

Closes the default-cutover portion of #1370.