refactor: extract zeph-orchestration crate from zeph-core

[bug-ops]

Epic: #1973

Extract crates/zeph-core/src/orchestration/ (5,384 LOC) into standalone zeph-orchestration crate.

Modules: TaskGraph, DagScheduler, LlmPlanner, LlmAggregator, AgentRouter, command.rs

Acceptance Criteria:

• zeph-orchestration compiles independently
• zeph-core depends on zeph-orchestration
• All orchestration tests pass in new crate
• Feature flag orchestration moves to new crate

Complexity: Medium | Effort: 4h
Blocks: All EPIC-01 children depend on this or are independent