feat(web): add Turn timeline and usage

[sam2tom]

Summary

• load every Turn page independently of Session/Item recovery
• reconcile exact Turn lifecycle SSE snapshots without regressing terminal state
• show accessible lifecycle status, server timing, safe errors, linked Items, and scoped Usage
• pin the Parsar Turn/Usage fixture and document pagination, recovery, tracing, and unavailable-value boundaries
• preserve the canonical Environment UUID identity behavior now on main

Acceptance evidence

• pagination follows limit=100&order=asc, rejects repeated/cyclic cursors and cross-Session Turns
• queued, in-progress, waiting, completed, failed, and cancelled states have accessible labels
• completed and running durations use server timestamps; missing/invalid values render Unknown
• errors do not hide persisted Items; Items associate only through protocol turn_id
• input, output, total, cached, and reasoning token counts remain distinct for Session and per-Turn Usage
• stale reads, Session switching, terminal SSE refresh, durable/live races, failed timeline reads, and narrow layout are covered

Latest-main merge

• tested head: 0e17eddfcc6ff693ae6ac4ab78d17933002c1248
• tested base: 5aa90e4dad33948e7efc66922a626745ac5cb806
• latest main was integrated with an ordinary merge commit; no rebase or force push
• origin/main is an ancestor of the tested head
• diff against main is exactly the original 16 Issue [agent] Add Turn timeline, real elapsed time, errors, and Usage #7 paths

Validation

• focused UUID + Turn tests: agents-client 42/42; Web 49/49
• pnpm check with repository-pinned pnpm 10.30.3: root 4/4, agents-client 46/46, Web 166/166; typecheck and build passed
• AGENTS_FIXTURE_PORT=18112 AGENTS_WEB_PORT=4192 pnpm test:acceptance: 15/15 passed
• git diff --check origin/main...HEAD: passed
• unmerged-index and private-state tracked-file audits: passed
• local report: /private/tmp/agents-core-web-pr20-merge-acceptance.md (intentionally untracked)

Boundaries

• no Parsar Core changes, paid calls, deployment, service shutdown, branch deletion, or Worktree deletion
• remains Draft pending the separately authorized Ready/merge step

Closes #7