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

roboco/models/session.py

@@ -3,9 +3,15 @@ Session Model
 
 Sessions group messages within boundaries (time, count, content length).
 They are automatically created and closed based on configuration.
+
+Session-Task Relationships:
+    PMs can create work sessions as discussion contexts for tasks.
+    A session can discuss multiple related tasks.
+    A task can have multiple sessions (planning, review, retrospective).
 """
 
 from datetime import UTC, datetime, timedelta
+from enum import StrEnum
 from uuid import UUID, uuid4
 
 from pydantic import Field
@@ -16,6 +22,44 @@ from roboco.models.base import (
     TimestampMixin,
 )
 
+# =============================================================================
+# SESSION SCOPE (Context Level)
+# =============================================================================
+
+
+class SessionScope(StrEnum):
+    """
+    Scope level for sessions - determines context loading strategy.
+
+    Sessions at different scopes serve different purposes:
+    - INITIATIVE: Cross-cell coordination (Main PM, #dev-all)
+    - CELL: Cell-specific work (Cell PM, #backend-cell)
+    - TASK: Individual task execution (Developer level)
+
+    When loading context for an agent:
+    - Load their scope's sessions fully
+    - Load parent scope sessions as summaries/references
+    """
+
+    INITIATIVE = "initiative"  # Cross-cell, Main PM level
+    CELL = "cell"  # Cell-specific, Cell PM level
+    TASK = "task"  # Individual task execution
+
+
+# =============================================================================
+# SESSION-TASK RELATIONSHIP TYPES
+# =============================================================================
+
+
+class SessionTaskRelationshipType(StrEnum):
+    """Type of relationship between a session and a task."""
+
+    DISCUSSION = "discussion"  # General discussion about the task
+    PLANNING = "planning"  # Planning session for the task
+    REVIEW = "review"  # Review/retrospective session
+    RETROSPECTIVE = "retrospective"  # Post-completion reflection
+
+
 # =============================================================================
 # SUPPORTING MODELS
 # =============================================================================
@@ -72,6 +116,12 @@ class Session(TimestampMixin):
     # State
     status: SessionStatus = Field(default=SessionStatus.ACTIVE)
 
+    # Scope (for smart context loading)
+    scope: SessionScope = Field(
+        default=SessionScope.TASK,
+        description="Session scope level - initiative, cell, or task",
+    )
+
     # Timestamps
     started_at: datetime = Field(default_factory=lambda: datetime.now(UTC))
     last_activity_at: datetime = Field(default_factory=lambda: datetime.now(UTC))
@@ -97,3 +147,65 @@ class SessionCreate(RobocoBase):
 
     group_id: UUID
     config: SessionConfig | None = None
+
+
+# =============================================================================
+# SESSION-TASK LINK MODELS
+# =============================================================================
+
+
+class SessionTaskLink(TimestampMixin):
+    """
+    Represents a link between a session and a task.
+
+    Used for reading/displaying session-task relationships.
+    """
+
+    id: UUID = Field(default_factory=uuid4, description="Link ID")
+    session_id: UUID = Field(..., description="Session ID")
+    task_id: UUID = Field(..., description="Task ID")
+
+    # Relationship metadata
+    is_primary: bool = Field(
+        default=False, description="Is this the primary discussion session for the task"
+    )
+    relationship_type: SessionTaskRelationshipType = Field(
+        default=SessionTaskRelationshipType.DISCUSSION,
+        description="Type of session-task relationship",
+    )
+
+    # Audit
+    added_at: datetime = Field(default_factory=lambda: datetime.now(UTC))
+    added_by: UUID | None = Field(default=None, description="PM who created the link")
+
+
+class SessionTaskLinkCreate(RobocoBase):
+    """Schema for creating a session-task link."""
+
+    session_id: UUID = Field(..., description="Session to link")
+    task_id: UUID = Field(..., description="Task to link")
+    is_primary: bool = Field(
+        default=False, description="Mark as primary session for this task"
+    )
+    relationship_type: SessionTaskRelationshipType = Field(
+        default=SessionTaskRelationshipType.DISCUSSION,
+        description="Type of relationship",
+    )
+
+
+class SessionForTasksCreate(RobocoBase):
+    """Schema for PM creating a session linked to multiple tasks."""
+
+    task_ids: list[UUID] = Field(..., min_length=1, description="Tasks to link")
+    channel_slug: str = Field(..., description="Channel where session is created")
+    scope: SessionScope = Field(
+        default=SessionScope.CELL,
+        description="Session scope level for context loading strategy",
+    )
+    config: SessionConfig | None = Field(
+        default=None, description="Session boundary configuration"
+    )
+    relationship_type: SessionTaskRelationshipType = Field(
+        default=SessionTaskRelationshipType.DISCUSSION,
+        description="Relationship type for all links",
+    )