fix(server): keep agent timelines in memory (#3647)

The filesystem timeline store rewrote each agent's full transcript on every buffered update, causing write amplification that grew with conversation length. Return production timelines to runtime memory and provider-history hydration while preserving the dormant storage interface for a future intentional design.

docs/architecture.md

@@ -327,7 +327,7 @@ initializing → idle ⇄ running
   client-side dedup; the default fetch page is 200 items.
 - Timeline row `timestamp` values are canonical daemon-owned timestamps. Providers may supply original replay timestamps, but clients must not guess timestamp trust or hide time UI based on local clock heuristics.
 - Events stream to connected clients in real time; correctness is backed by authoritative timeline fetches and paged-to-completion catch-up.
-- Agent state persists to `$PASEO_HOME/agents/{cwd-with-dashes}/{agent-id}.json` (timeline rows live alongside the record). That storage path is derived from `cwd`, not from workspace id.
+- Agent state persists to `$PASEO_HOME/agents/{cwd-with-dashes}/{agent-id}.json`. Timeline rows are runtime memory; provider history is the durable transcript authority and resumed agents rebuild from it. That storage path is derived from `cwd`, not from workspace id.
 
 ## Right-sidebar boundary: directory-backed vs workspace-owned
 
@@ -402,7 +402,7 @@ Providers that can accept native tool definitions should set `supportsNativePase
 
 ```
 $PASEO_HOME/
-├── agents/{cwd-with-dashes}/{agent-id}.json   # Agent record + persisted timeline rows
+├── agents/{cwd-with-dashes}/{agent-id}.json   # Agent record
 ├── projects/projects.json                      # Project registry
 ├── projects/workspaces.json                    # Workspace registry
 ├── projects/icons/                             # Custom project icon images