refactor: unify duplicated chat-recovery/repair machinery into the shared agents/chat layer (N3)

[threepointone]

Problem

The chat-recovery + transcript-repair machinery is duplicated, nearly verbatim, between packages/think/src/think.ts and packages/ai-chat/src/index.ts. Every recovery fix has to be written twice, the two copies drift, and the drift is a real source of bugs and confusion.

This was painful and concrete across the recent recovery stack (#1633, #1634, #1635, #1636, #1638, #1640, #1641): each touched think and ai-chat in lockstep, and the divergence caused issues — e.g. ai-chat's persist gate is streamStillActive-only while think uses _shouldPersistOrphanedPartial (an intentional asymmetry, but one that has to be re-reasoned every time), and ai-chat has no transcript-repair pass at all while think does.

Goal

Hoist the shared recovery/repair machinery into packages/agents/src/chat/ so it is written once and both Think and AIChatAgent consume it. Pure refactor — no behavior change. Design home: design/chat-shared-layer.md.

Duplicated surface to unify (audit, not exhaustive)

• Budget / incident: _beginChatRecoveryIncident, _handleInternalFiberRecovery, _exhaustChatRecovery, _sweepStaleChatRecoveryIncidents, the ChatRecoveryIncident shape, and the constants (CHAT_RECOVERY_NO_PROGRESS_WINDOW_MS, CHAT_RECOVERY_ALARM_DEBOUNCE_MS, CHAT_RECOVERY_MAX_WINDOW_MS, DEFAULT_CHAT_RECOVERY_MAX_ATTEMPTS, DEFAULT_CHAT_RECOVERY_STABLE_TIMEOUT_MS, CHAT_RECOVERY_PROGRESS_KEY, AGENT_TOOL_STREAM_PROGRESS_BUMP_THROTTLE_MS).
• Progress signal: _chatRecoveryProgressMarker, _bumpChatRecoveryProgress, and the production-time bump sites (_storeChunkDurably in think / _storeStreamChunk in ai-chat) + the N9 _onAgentToolStreamProgress override.
• Orphan persistence: _persistOrphanedStream, _shouldPersistOrphanedPartial, _partialHasSettledToolResults, _hasPersistedRecoveredAssistant.
• Known intentional asymmetries to reconcile (decide: unify or document at the seam):
  • ai-chat persist gate is streamStillActive-only vs think's streamStillActive || (streamIsTerminal && !alreadyPersisted).
  • Transcript repair is think-only (_repairTranscriptForProvider, _repairToolTranscriptParts, repairInterruptedToolPart, _toolPartHasSettledResult, _normalizeToolInput) — ai-chat has none. Decide whether the shared layer offers it to both or stays think-only by design.

Constraints / sequencing

• Do this AFTER the recovery PR stack landed (it has — fix(think,ai-chat): make chat-recovery progress signal compaction-immune (#1628) #1633/fix(think,ai-chat): preserve settled work when a recovery turn is given up (#1631) #1634/feat(think): add repairInterruptedToolPart hook for client-resolved tools (#1631) #1635/feat(think,ai-chat,agents): expose incident identity + enrich onExhausted payload (#1631) #1636/fix(think,ai-chat): wall-clock-keyed-to-progress recovery budget + alarm debounce (#1637) #1638/fix(agents,think,ai-chat): re-attach to still-running sub-agent runs on parent recovery (#1630) #1640/fix(agents,think,ai-chat): count a sub-agent's progress as the orchestrating parent's recovery progress (N9) #1641 are all on main), to avoid massive rebase conflicts. Do it BEFORE piling more recovery work (discussion: route a stream-stall watchdog abort into bounded recovery instead of a terminal error #1626 watchdog→recovery, Surface a live "recovering…" status to chat clients during durable recovery #1620 recovering-status, durable telemetry) on top, so those land write-once instead of twice. (Note: the team has chosen to land discussion: route a stream-stall watchdog abort into bounded recovery instead of a terminal error #1626/Surface a live "recovering…" status to chat clients during durable recovery #1620/telemetry first, then this refactor before the formal release — so this issue is the "after those three" checkpoint.)
• Pure refactor: full think + ai-chat suites must stay green with no test changes beyond import paths; behavior identical.
• Watch the gotcha: editing packages/agents/src/chat/* requires nx run agents:build before think/ai-chat typecheck (they compile against built dist).

Acceptance

• Recovery/repair logic lives once in packages/agents/src/chat/; Think and AIChatAgent delegate to it.
• npm run check green; full suites green with no behavior change; no new public API.