feat: add session archive state and scoped stats

[teedole]

Summary

Add archive/delete session state with subtree-aware visibility, deterministic re-ingest tombstones, and archive-aware session UI controls. Extend stats APIs and cards so project, tool, text, and archived filters describe the same visible session set.

This is stacked on #73 because it advances that schema baseline from v19 to v20. Retarget to main after #73 lands.

Why

This completes WS-A and WS-E from the UX/API hardening plan: users can trust filtered summaries, hide work without losing exact replay, and permanently delete a full session subtree without a later sync resurrecting it.

Validation

• just check passes: 683 tests, TypeScript, Biome, and darwin-arm64 distribution staging smoke.
• New behavior has focused tests. No existing test was weakened or deleted to make this pass.
• Two adversarial review passes completed with all findings fixed and a clean second read.
• Rendered at 1440px light and 680px dark; verified archive, inherited archive, unarchive, delete dialog keyboard/focus behavior, URL preservation, and matching cards/rows.
• Restarted against a synthetic fixture source and verified manual sync skips a deleted session instead of resurrecting it.

Archive schema

The baseline is LATEST_SCHEMA_VERSION in src/db.ts. Check one:

• This change does not touch the archive schema.
• This change bumps the schema. It adds schema v20 and the session_user_state migration. Supported TypeScript-cutover archives migrate in place; pre-v8 archives remain rebuild-only.

[gemini-code-assist]

Code Review

This pull request introduces a durable session archiving and deletion feature to prevent deleted sessions from being resurrected during synchronization. It adds a new session_user_state table (schema version 20), updates database queries, statistics, and recommendations to respect these states, and provides UI controls for archiving, unarchiving, and deleting sessions. Feedback on the changes highlights a redundant check for deleted session identities in src/ingest.ts that duplicates logic already handled by writeSession.

[teedole]

Validator follow-up complete and rebased at signed tip 6684ac9.

• Late Codex and Claude descendants inherit deleted lineage transitively and order-independently, including deleted intermediate Claude boundaries.
• Empty session_user_state archives bypass recursive visibility work across every archive-wide rollup.
• Scoped Sessions stat cards include reload state, while the sidebar summary remains unscoped.

Evidence: focused state/ingest/stats suite 131/131; adversarial re-review clean; combined gate 781/781. A headed browser canary changed a scratch session state under include_archived=true and observed a second scoped summary request after archive_updated before restoring the row to visible.