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

alembic/versions/001_initial_schema.py

@@ -10,6 +10,7 @@ Creates all tables for the RoboCo AI Agents Company system:
 - channels: Communication channels
 - groups: Role-based groups within channels
 - sessions: Bounded message sessions
+- session_tasks: Many-to-many session-task links (PM work sessions)
 - messages: Extracted messages
 - notifications: Formal notifications
 - journals: Agent personal logs
@@ -292,6 +293,13 @@ def upgrade() -> None:
             server_default="active",
             index=True,
         ),
+        sa.Column(
+            "scope",
+            sa.Enum("initiative", "cell", "task", name="sessionscope"),
+            nullable=False,
+            server_default="task",
+            index=True,
+        ),
         sa.Column(
             "started_at", sa.DateTime(), nullable=False, server_default=sa.func.now()
         ),
@@ -309,6 +317,54 @@ def upgrade() -> None:
         ),
     )
 
+    # ==========================================================================
+    # SESSION_TASKS TABLE (Many-to-Many Junction)
+    # ==========================================================================
+    op.create_table(
+        "session_tasks",
+        sa.Column("id", postgresql.UUID(as_uuid=True), primary_key=True),
+        sa.Column(
+            "session_id",
+            postgresql.UUID(as_uuid=True),
+            sa.ForeignKey("sessions.id", ondelete="CASCADE"),
+            nullable=False,
+            index=True,
+        ),
+        sa.Column(
+            "task_id",
+            postgresql.UUID(as_uuid=True),
+            sa.ForeignKey("tasks.id", ondelete="CASCADE"),
+            nullable=False,
+            index=True,
+        ),
+        sa.Column("is_primary", sa.Boolean(), nullable=False, server_default="false"),
+        sa.Column(
+            "relationship_type",
+            sa.String(50),
+            nullable=False,
+            server_default="discussion",
+        ),
+        sa.Column(
+            "added_at", sa.DateTime(), nullable=False, server_default=sa.func.now()
+        ),
+        sa.Column(
+            "added_by",
+            postgresql.UUID(as_uuid=True),
+            sa.ForeignKey("agents.id", ondelete="SET NULL"),
+            nullable=True,
+        ),
+        sa.UniqueConstraint("session_id", "task_id", name="uq_session_task"),
+    )
+
+    # Partial unique index: only one primary session per task
+    op.execute(
+        """
+        CREATE UNIQUE INDEX ix_session_tasks_primary_per_task
+        ON session_tasks (task_id)
+        WHERE is_primary = true
+        """
+    )
+
     # ==========================================================================
     # MESSAGES TABLE
     # ==========================================================================
@@ -651,6 +707,8 @@ def downgrade() -> None:
     op.drop_table("journals")
     op.drop_table("notifications")
     op.drop_table("messages")
+    op.drop_index("ix_session_tasks_primary_per_task", table_name="session_tasks")
+    op.drop_table("session_tasks")
     op.drop_table("sessions")
     op.drop_table("groups")
     op.drop_table("channels")
@@ -668,6 +726,7 @@ def downgrade() -> None:
     op.execute("DROP TYPE IF EXISTS notificationtype")
     op.execute("DROP TYPE IF EXISTS messagetype")
     op.execute("DROP TYPE IF EXISTS sessionstatus")
+    op.execute("DROP TYPE IF EXISTS sessionscope")
     op.execute("DROP TYPE IF EXISTS channeltype")
     op.execute("DROP TYPE IF EXISTS complexity")
     op.execute("DROP TYPE IF EXISTS taskstatus")