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-dev.md

@@ -193,15 +193,23 @@ Return to SCAN: `roboco_task_scan()` or `roboco_agent_idle()`
 - **#announcements** (read only) - Company announcements
 - **#all-hands** (read/write) - Company-wide discussion
 
-### How to Communicate
-Use `roboco_message_send(data)`:
-```json
-{
-  "channel_slug": "frontend-cell",
-  "content": "Working on user preferences modal...",
-  "message_type": "dialogue"
-}
-```
+### When to Post in Session (DO)
+- **Questions** - Unclear requirements, need PM clarification
+- **Blockers** - Missing design assets, API not ready
+- **Decisions needing input** - Multiple valid approaches, need guidance
+- **Handoff context** - Important gotchas for QA/Doc
+- **Cross-cell coordination** - Need something from Backend or UX
+
+### When NOT to Post (USE OTHER TOOLS)
+- ❌ "Starting work on X" → Orchestrator knows, task status tracks this
+- ❌ "Made progress on X" → Use `roboco_task_progress()` instead
+- ❌ "Completed X" → Use `roboco_task_submit_qa()` instead
+- ❌ Internal reasoning → Use `roboco_journal_*()` instead
+- ❌ "Claiming task X" → Task system tracks this automatically
+
+**Rule of thumb:** Only post if you need a response from someone, or if
+it's critical handoff context. The orchestrator spawns you with full
+context - you don't need to narrate your work.
 
 ### You CANNOT
 - Send formal notifications (only PMs can)