Parent: #1235
Blocked by: #1236

Summary

Implement the DAG scheduler that drives parallel task execution through existing SubAgentManager infrastructure.

Branch: feat/m33/orchestration-scheduler

Deliverables

New files

• crates/zeph-core/src/orchestration/scheduler.rs — DagScheduler, SchedulerAction, TaskEvent, TaskOutcome
• crates/zeph-core/src/orchestration/router.rs — AgentRouter trait + RuleBasedRouter impl

Modified files

• crates/zeph-core/src/subagent/manager.rs — add completion_tx: Option<mpsc::Sender<TaskEvent>> to SubAgentHandle, fire on loop termination. Add spawn_for_task() method.

Key design decisions

• ADR-026: Command pattern — scheduler produces SchedulerAction values, caller executes against manager. No &mut SubAgentManager held across await points.
• ADR-027: Single mpsc::Sender<TaskEvent> channel for all agent completions (not per-agent watch receivers).
• Cross-task context: <completed-dependencies> block injected into task prompt with dependency_context_budget (16384 chars total, divided equally across deps).
• Task timeout: wall-clock monitoring via task_timeout_secs config.
• Concurrency: respects both max_parallel and SubAgentManager.max_concurrent.

Scheduler tick loop

1. Drain pending TaskEvents from event_rx
2. Update task statuses, call ready_tasks() for newly schedulable tasks
3. Route ready tasks to agents via AgentRouter
4. Emit SchedulerAction::Spawn for each (up to concurrency limit)
5. If no running/ready tasks and all terminal → Done

Router fallback chain

1. task.agent_hint exact match against loaded definitions
2. Tool requirement matching (future: task keywords → agent tool policy)
3. First available agent (fallback)

Tests (~20)

• Linear chain executes sequentially (via mock manager)
• Independent tasks produce parallel Spawn actions
• Completion triggers downstream scheduling
• Failure + Abort produces Cancel actions for all running tasks
• Failure + Skip propagates correctly through dependents
• Failure + Retry re-schedules task (resets to Ready)
• Concurrency limit respected in action generation
• All tasks completed → Done with Completed status
• Router: agent_hint exact match
• Router: fallback to first available
• Task timeout produces Cancel action
• Manager spawn rejection (concurrency) keeps task Ready

Dependencies

Blocked by: Phase 1 (#1236)
Can run in parallel with: Phase 2 (#1237)