feat(web): add Session metadata and delete flows

[sam2tom]

Summary

• add an accessible Session details surface for title and string metadata editing
• reconcile the latest durable Session before updates while preserving and safely rebasing drafts
• require exact delete confirmation with deterministic selected/inactive Session cleanup
• reconcile uncertain delete results with one read-only GET and never retry DELETE automatically
• abort and fence selected Session, Item, Turn, Environment, and SSE continuations
• fail closed on wrong-ID or malformed Session reads before update or delete reconciliation

Latest-main validation

• tested head: b4a751fedf1b2b65fdbf20005603acac68ed85b9
• tested base: bac551cadaed45fc60ad7fa0a01020d47ff3856a
• pnpm check (root 4, agents-client 47, Web 187; typecheck and build passed)
• full Playwright acceptance on isolated ports (28 passed)
• focused Session guard unit tests (18 passed)
• focused wrong-ID/malformed browser regression (1 passed)
• git diff --check origin/main...HEAD
• exact 12-path Issue [agent] Add Session rename, metadata, and delete flows #6 diff and private-state audit passed
• independent read-only review found no merge-blocking findings

Integration

Latest main was merged normally into the Issue branch in 634539112a2884bd557a833abfd4567bf114b65a; no rebase or force push was used. The final complete Session guard is b4a751fedf1b2b65fdbf20005603acac68ed85b9. Review the PR diff against main as the Issue #6-only delta.

No deployment, paid model/provider request, branch deletion, or Worktree deletion was performed.

Closes #6