feat(subagent): phase1 - persistent subagent instance types and registry

[ArrogHie]

Summary

Phase 1 of Persistent Subagent + Structured Output: define the orchestrator-level SubagentInstance data model and an in-memory instance registry. No existing call paths are modified.

Changes

• subagent_instance.rs (new): SubagentInstance type mapping a resumable instance_id to a child Runtime Session, SubagentInstanceStatus lifecycle enum (Running / Idle / Destroyed), and a DashMap-backed SubagentInstanceRegistry.
• Registry operations: register / get / set_running / set_idle / destroy / destroy_all_for_parent / list_for_parent / active_count, with English logs and validated state transitions (invalid transitions return Err with a warn! log).
• coordination/mod.rs: crate-wide export of the new types (pub(crate)), following the internal-helper-module convention (mod + pub(crate) use).
• Unit tests: 16 tests covering storage, all state-transition paths (valid and invalid), destroy semantics, per-parent listing/cleanup, can_resume, instance ID uniqueness, and active_count.

Verification

• pnpm run fmt:rs - formatted
• cargo check -p bitfun-core - passes, no new warnings
• cargo test -p bitfun-core subagent_instance - 16 passed
• cargo test -p bitfun-core coordination - 160 passed (no regressions)

Notes

• Phase 2 will wire the registry into the Coordinator and add create/resume APIs; TaskTool changes are deferred to Phase 5.
• The types are currently unreferenced in production call paths by design (#[expect(unused_imports)] / #[allow(dead_code)] on the module, with comments), which the compiler will flag for removal once Phase 2 lands.