feat: Implement council brief processing loop for task execution

Add processBriefs() polling loop that checks for assigned council briefs
and executes them using the ExecutionEngine infrastructure.

Changes:
- Add GetCurrentAssignment() public method to council.Manager
- Make HTTPServer.CouncilManager public for brief access
- Add processBriefs() 15-second polling loop in agent_support.go
- Add executeBrief() to initialize and run ExecutionEngine
- Add buildExecutionRequest() to convert briefs to execution requests
- Add uploadResults() to send completed work to WHOOSH
- Wire processBriefs() into StartAgentMode() as background goroutine

This addresses the root cause of task execution not happening: briefs
were being stored but never polled or executed. The execution
infrastructure (ExecutionEngine, AI providers, prompt system) was
complete but not connected to the council workflow.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

internal/council/manager.go

@@ -433,6 +433,11 @@ func (m *Manager) currentAssignmentSnapshot() *RoleAssignment {
 	return m.currentAssignment
 }
 
+// GetCurrentAssignment returns the current role assignment (public accessor)
+func (m *Manager) GetCurrentAssignment() *RoleAssignment {
+	return m.currentAssignmentSnapshot()
+}
+
 // roleClaimResponse mirrors WHOOSH role claim response payload.
 type roleClaimResponse struct {
 	Status        string        `json:"status"`