feat: session refactor (#30)

AGENTS.md

@@ -33,7 +33,7 @@ make -C apps/backend lint
 
 ## Backend Architecture (High Level)
 
-- **Transport**: WebSocket-only for client <-> backend.
+- **Transport**: WebSocket for realtime updates; HTTP for CRUD + SSR fetches.
 - **Gateway**: Single WS endpoint at `ws://localhost:8080/ws`.
 - **Orchestrator**: Starts/stops/resumes tasks, manages sessions.
 - **Agent lifecycle**:
@@ -92,7 +92,11 @@ Client (WS)             Backend                Runtime              agentctl
 
 ### Subscription Strategy
 
-TODO
+- Components subscribe to the channels they need (task, session, repo), using hooks.
+- WebSocket client deduplicates subscriptions with reference counting.
+- Reconnects re-subscribe all active channels automatically.
+- Event handlers update store slices; components only read from store.
+- URL params set active task/session in store for consistent view state.
 
 **Why This Works**
 
@@ -111,15 +115,30 @@ Layer 3: Components (subscribe + render)
 
 ### Store Structure
 
-TODO
+- Task/session scoped data is keyed in store:
+  - `messages.bySession[sessionId]`
+  - `messages.metaBySession[sessionId]`
+  - `gitStatus` keyed by `sessionId`
+  - `shell.outputs` keyed by `sessionId`
+- Active selection state lives in store:
+  - `tasks.activeTaskId`
+  - `tasks.activeSessionId`
 
 ### Custom Hooks Pattern
 
-TODO
+- Hooks own subscriptions and return data from store:
+  - `useSessionMessages(sessionId)` → messages + loading/meta
+  - `useLazyLoadMessages(sessionId)` → pagination
+  - `useSessionGitStatus(sessionId)` → git state
+  - `useSessionShellOutput(sessionId)` → shell output
+- Hooks subscribe on mount, cleanup on unmount; no manual dedup in components.
 
 ### Page-Level Responsibilities
 
-TODO
+- SSR fetches initial task/session + board snapshot, hydrates store.
+- Page reads URL `sessionId`, resolves task via session, and calls `setActiveSession(taskId, sessionId)` after load.
+- Page coordinates task list + session list loading (e.g., `listTaskSessions`).
+- Components render from store and trigger hooks for live updates.
 
 ## Agents and ACP