storkit: create 415_refactor_split_agents_pool_mod_rs_into_submodules

.storkit/work/1_backlog/415_refactor_split_agents_pool_mod_rs_into_submodules.md

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+---
+name: "Split agents/pool/mod.rs into submodules"
+---
+
+# Refactor 415: Split agents/pool/mod.rs into submodules
+
+## Current State
+
+- TBD
+
+## Desired State
+
+Refactor the monolithic server/src/agents/pool/mod.rs (2407 lines) into focused submodules within the pool/ directory.
+
+## Acceptance Criteria
+
+- [ ] types.rs contains StoryAgent, PendingGuard, AgentInfo, composite_key, and related helper structs
+- [ ] lifecycle.rs contains start_agent, stop_agent, wait_for_agent and their unit tests
+- [ ] worktree.rs contains create_worktree, get_project_root, find_active_story_stage and their unit tests
+- [ ] query.rs contains list_agents, available_agents_for_stage, get_log_info, subscribe, drain_events and their unit tests
+- [ ] process.rs contains kill_all_children, kill_child_for_key, ChildKiller registry methods and their unit tests
+- [ ] test_helpers.rs contains inject_test_agent and its variants (4 methods)
+- [ ] mod.rs contains AgentPool struct, new(), and re-exports all public types
+- [ ] Unit tests live in their respective module files, not in a separate tests module
+- [ ] No public API changes — all existing imports continue to work
+
+## Out of Scope
+
+- TBD