Epic: #1973

Extract crates/zeph-core/src/subagent/ (9,108 LOC) into standalone zeph-subagent crate.

Modules: SubAgentManager, grants, def, transcripts.

Acceptance Criteria:

• zeph-subagent compiles independently
• Subagent tests pass in new crate
• Agent interacts via trait, not direct struct access

Complexity: Medium-Large | Effort: 6h
Note: Subagent module has deep coupling to Agent — may need interface trait.