Parent: #1235

Summary

Implement core type system, DAG algorithms, and SQLite persistence for task orchestration.

Branch: feat/m33/orchestration-types
Feature flag: orchestration (optional, not default)

Deliverables

New files

• crates/zeph-core/src/orchestration/mod.rs — module declaration, feature-gated
• crates/zeph-core/src/orchestration/graph.rs — TaskGraph, TaskNode, TaskId, TaskStatus, GraphStatus, TaskResult, FailureStrategy, GraphId, GraphStore trait
• crates/zeph-core/src/orchestration/dag.rs — validate, toposort, ready_tasks, propagate_failure
• crates/zeph-core/src/orchestration/error.rs — OrchestrationError enum
• crates/zeph-core/src/orchestration/config.rs — OrchestrationConfig
• crates/zeph-memory/src/sqlite/graph_store.rs — SqliteGraphStore impl
• crates/zeph-memory/migrations/021_task_graphs.sql — SQLite schema

Modified files

• crates/zeph-core/src/lib.rs — #[cfg(feature = "orchestration")] pub mod orchestration
• crates/zeph-core/src/config/types.rs — add orchestration field to Config
• crates/zeph-core/Cargo.toml — add orchestration feature
• Cargo.toml root — feature propagation
• crates/zeph-memory/src/sqlite/mod.rs — pub mod graph_store (feature-gated)
• crates/zeph-memory/Cargo.toml — orchestration feature

Key design decisions

• ADR-021: Persistence in zeph-memory, trait in zeph-core (zeph-core never depends on sqlx)
• TaskId(u32) indexes into Vec<TaskNode> (not HashMap — O(1) access, max_tasks=20)
• Edges implicit in TaskNode.depends_on (no separate edge list)
• Only Completed unblocks downstream; Skipped triggers transitive skip propagation
• GraphStore trait: save_graph, load_graph, list_graphs, delete_graph

DAG algorithms

1. Cycle detection: Kahn's algorithm — simultaneously validates acyclicity and produces topological sort
2. Ready tasks: scan for Pending tasks with all deps Completed
3. Failure propagation: Abort (cancel all), Skip (transitive), Retry (reset to Ready), Ask (pause)
4. Validation: valid TaskId refs, no self-references, cycle-free, at least one root

Tests (~35)

• Cycle detection: single cycle, multi-node cycle, no cycle, self-reference
• Topological sort: linear chain, diamond, wide parallel, single node, empty graph
• Ready tasks: initial roots, after completion, skipped does NOT unblock
• Failure propagation: abort cascades, skip cascades transitively, retry resets status
• Validation: invalid TaskId references, empty graph, single task
• Serialization round-trip for all types (serde JSON)
• Persistence: save/load round-trip, list, delete (test SQLite)
• Display impls for user-facing types

Dependencies

None — first phase.