feat(sessions): Session-Task linking with scoped context management

Major feature: Sessions are now linked to tasks with smart routing and context loading, ensuring agents have proper discussion context.

## Session-Task Relationship (Many-to-Many)
  - Added SessionTaskTable junction table linking sessions to tasks
  - Sessions can link to multiple tasks, tasks can have multiple sessions
  - is_primary flag marks the main discussion session for a task
  - relationship_type: discussion, planning, review, retrospective
  - Subtasks auto-inherit parent task's session

## BACKLOG Status + Activation Flow
  - Tasks now created with BACKLOG status (not PENDING)
  - PMs must create session BEFORE activating task
  - roboco_task_activate() transitions BACKLOG → PENDING
  - Prevents race condition where dev starts before session exists
  - Flow: CREATE (backlog) → SESSION → ACTIVATE (pending) → spawn

## Session Scopes
  - SessionScope enum: initiative, cell, task
  - initiative: Cross-cell coordination (Main PM, #dev-all)
  - cell: Cell-specific work (Cell PM default)
  - task: Individual task execution (dev level)
  - Enables future smart context loading by scope

## Message Routing to Task Sessions
  - When task_id provided in roboco_message_send(), routes to task's primary session instead of channel's active session
  - New API endpoint: GET /sessions/for-task/{task_id}
  - TaskResponse now includes linked sessions array

## Dev Session Access
  - New tool: roboco_session_history_for_task(task_id)
  - Devs can now see their task's discussion history
  - Messages tagged with task_id for filtering

## Communication Guidelines
  - Added "When to Post / When NOT to Post" to all 9 agent blueprints
  - Devs/QA/Doc should use task tools for status, journal for reasoning
  - Sessions reserved for coordination that needs response
  - Reduces noise: no "Starting work" or "Made progress" chat messages

Files changed:
  - DB: SessionTaskTable, SessionScope column
  - Services: messaging.py (linking), task.py (activation)
  - MCP: 5 new session tools, message routing update
  - API: session-task endpoints, TaskResponse sessions
  - Blueprints: All 13 updated with session/activation workflow

agents/blueprints/frontend/fe-pm.md

@@ -46,6 +46,12 @@ You interact with RoboCo systems through MCP tools:
 - `roboco_task_assign(task_id, agent_slug)` - Assign task to an agent
 - `roboco_task_complete(task_id)` - Complete a parent task after subtasks done
 
+**Session Management (Work Sessions for Tasks):**
+- `roboco_session_create_for_tasks(data)` - Create a work session linked to tasks
+- `roboco_session_link_task(data)` - Link additional task to existing session
+- `roboco_session_unlink_task(session_id, task_id)` - Remove task from session
+- `roboco_session_get_for_task(task_id)` - Get sessions linked to a task
+
 **Journal (Document Your Thinking):**
 - `roboco_journal_entry(data)` - General journal entry
 - `roboco_journal_reflect(data)` - Task reflection
@@ -149,6 +155,42 @@ roboco_task_assign("{task_id}", "fe-dev-1")
 - Every subtask MUST have both `parent_task_id` AND `assigned_to`
 - Do NOT keep tasks for yourself - delegate to developers!
 
+### 7.5. CREATE WORK SESSION (REQUIRED)
+**Tool:** `roboco_session_create_for_tasks(data)`
+
+After delegating, you MUST create a work session for the task:
+```python
+roboco_session_create_for_tasks({
+    "task_ids": ["task-uuid-1", "task-uuid-2"],
+    "channel_slug": "frontend-cell",
+    "scope": "cell",                             # Cell-level session
+    "relationship_type": "discussion"
+})
+```
+
+**Session scopes:**
+- `initiative` - Cross-cell coordination (Main PM only, #dev-all)
+- `cell` - Cell-specific work (your default, #frontend-cell)
+- `task` - Individual task execution (developer level)
+
+**Why sessions are mandatory:**
+- Every task needs a discussion context
+- QA and documenter see full context when reviewing
+- Subtasks auto-inherit parent task's primary session
+
+### 7.6. ACTIVATE TASK (REQUIRED)
+**Tool:** `roboco_task_activate(task_id)`
+
+After creating the session, activate the task:
+```python
+roboco_task_activate("task-uuid")
+```
+
+**Task flow:**
+```
+CREATE (backlog) → SESSION → ACTIVATE (pending) → Orchestrator spawns dev
+```
+
 ### 8. COMMUNICATE
 **Tool:** `roboco_message_send(data)`
 Tell the team what you did:
@@ -324,7 +366,12 @@ tools:
   # Task Management
   - roboco_task_scan, roboco_task_get, roboco_task_claim
   - roboco_task_start, roboco_task_plan, roboco_task_progress
-  - roboco_task_create, roboco_task_assign, roboco_task_complete
+  - roboco_task_create, roboco_task_assign, roboco_task_activate
+  - roboco_task_complete
+
+  # Session Management (REQUIRED before activation)
+  - roboco_session_create_for_tasks, roboco_session_link_task
+  - roboco_session_unlink_task, roboco_session_get_for_task
 
   # Journal
   - roboco_journal_entry, roboco_journal_decision