refactor: DB-driven agent output events with single emission point

DB log chunk insertion is now the sole trigger for agent:output events.
Eliminates triple emission (FileTailer, handleStreamEvent, output buffer)
in favor of: FileTailer.onRawContent → DB insert → EventBus emit.

- createLogChunkCallback emits agent:output after successful DB insert
- spawnInternal now wires onRawContent callback (fixes session 1 gap)
- Remove eventBus from FileTailer (no longer touches EventBus)
- Remove eventBus from ProcessManager constructor (dead parameter)
- Remove agent:output emission from handleStreamEvent text_delta
- Remove outputBuffers map and all buffer helpers from manager/handler
- Remove getOutputBuffer from AgentManager interface and implementations
- getAgentOutput tRPC: DB-only, no file fallback
- onAgentOutput subscription: no initial buffer yield, events only
- AgentOutputViewer: accumulates raw JSONL chunks, parses uniformly

docs/server-api.md

@@ -62,10 +62,10 @@ Each procedure uses `require*Repository(ctx)` helpers that throw `TRPCError(INTE
 | getAgent | query | Single agent by name or ID |
 | getAgentResult | query | Execution result |
 | getAgentQuestions | query | Pending questions |
-| getAgentOutput | query | Full output (DB chunks or file fallback) |
+| getAgentOutput | query | Full output from DB log chunks |
 | getActiveRefineAgent | query | Active refine agent for initiative |
 | listWaitingAgents | query | Agents waiting for input |
-| onAgentOutput | subscription | Live output stream + buffered history |
+| onAgentOutput | subscription | Live raw JSONL output stream via EventBus |
 
 ### Tasks
 | Procedure | Type | Description |