Issue 2341 Reliability Slice Plan

Scope: first PR from the consolidated issue triage. This PR should not try to solve the full backlog. It should remove a few high-confidence paper cuts from the first two buckets: install/startup contract and DB/export contract.

Phase 0: Documentation Discovery

Allowed APIs and patterns:

Install marker helpers live in src/npx-cli/install/setup-runtime.ts. Existing tests are in tests/setup-runtime.test.ts.
Runtime startup warning logic lives in plugin/scripts/version-check.js. It currently resolves the plugin root from CLAUDE_PLUGIN_ROOT, then from the script directory.
Export script reads worker settings via SettingsDefaultsManager.loadFromFile. Worker settings must respect CLAUDE_MEM_DATA_DIR, because shared path helpers and settings defaults already expose that environment override.
/api/sdk-sessions/batch is registered in src/services/worker/http/routes/DataRoutes.ts and expects memorySessionIds. Existing coercion tests are in tests/worker/http/routes/data-routes-coercion.test.ts.
Current PendingMessageStore writes and reads tool_use_id, but no longer reads worker_pid, retry_count, failed_at_epoch, or completed_at_epoch. Current schema guardrails should match code that runs today, not old migration intent.

Anti-pattern guards:

Do not reintroduce worker_pid in pending_messages unless the current claim query starts using it again.
Do not rely only on schema_versions for columns that current SQL references.
Do not add another install marker format. Read both legacy plain text and the current JSON format, but keep writing the JSON marker.
Do not make export-memories.ts fall back to ~/.claude-mem when CLAUDE_MEM_DATA_DIR is set.

Phase 1: Install Marker Compatibility

What to implement:

Teach readInstallMarker() to parse legacy plain-text marker files that only contain a version string.
Teach plugin/scripts/version-check.js to accept the same legacy marker shape.
Keep writeInstallMarker() unchanged so new installs write the canonical JSON schema.

Verification:

Add tests/setup-runtime.test.ts coverage for a plain-text .install-version.
Add a focused test for plugin/scripts/version-check.js behavior, or extend an existing plugin script test if one exists.
Run bun test tests/setup-runtime.test.ts.

Phase 2: Export Script Contract Repair

What to implement:

Update scripts/export-memories.ts to load settings from CLAUDE_MEM_DATA_DIR/settings.json instead of always using ~/.claude-mem/settings.json.
Change the /api/sdk-sessions/batch request body from sdkSessionIds to memorySessionIds.
Optionally allow DataRoutes to accept the legacy sdkSessionIds alias as a compatibility bridge, but prefer the canonical field in scripts.

Verification:

Add or update tests around the SDK-session batch route alias/coercion.
Add a script-level test if practical; otherwise verify by grep that scripts/export-memories.ts no longer sends sdkSessionIds and no longer hardcodes homedir(), '.claude-mem'.
Run the focused route/export tests.

Phase 3: Current Pending Queue Shape Guardrails

What to implement:

Add a regression test that initializes a DB whose schema_versions claims old pending-message migrations are applied while pending_messages.tool_use_id is missing. Constructing SessionStore should still add the missing column because current enqueue SQL requires it.
Add a regression test asserting the current fresh DB shape does not require worker_pid, since the current claim query does not use it.
If tests expose a real source/schema mismatch, update docs/schema comments to match current code rather than reintroducing unused columns.

Verification:

Run focused sqlite tests for SessionStore / PendingMessageStore.
Grep for live worker_pid reads in TypeScript before deciding whether it is still a required current column.

Final Verification

Run focused tests changed by this PR.
Run npm run typecheck:root if dependencies are available.
Run git diff --check.
Open a non-draft PR against the upstream default branch.
Do not merge, release, or ship without explicit user approval.