feat(web): add protocol-honest trace workbench

[sam2tom]

Summary

• add Conversation and Trace tabs while preserving conversation draft and scroll state
• add a searchable Turn and Item trace ledger with desktop split details and a mobile detail view
• move the complete Turn timeline into collapsed Trace diagnostics so Conversation stays focused on messages
• present missing per-Item timing as unavailable while preserving Core-reported tool and Turn durations
• hide the non-actionable environment:none Workspace placeholder while retaining fail-closed Environment states
• make every compatible Agent's Start Session action visible, with explicit unavailable reasons for blocked Agents
• show a composer-adjacent activity indicator only while Core reports the Session as in_progress

Protocol boundary

• pin upstream-dependent conclusions to Parsar revision d91ba48ac6c49cfdf6f08d7687b9be76ba6d53ee
• show an explicitly non-time-scaled Input / Model / Tools sequence overview
• do not infer per-Item timing, TTFT, throughput, Provider, Step, request hierarchy, or sub-agent hierarchy
• drive the conversation activity indicator only from Core's Session status, not local busy state, SSE connectivity, Items, or historical Turns
• document top-level Raw allowlisting and unavailable data explicitly

Validation

• pnpm check: root 67/67, agents-client 50/50, web 362/362; typecheck and production build passed
• full Playwright acceptance suite: 43/43 passed
• git diff --check passed
• GitHub Check / check passed on head 71db219

The production build retains the existing chunk-size warning; no new build failure was introduced.

Closes #26