Fix: reap stale agent pid files in poc-hook

scan_pid_files was removed as dead code but it was actually needed
by the hook path — the bug was that it was never wired in. Add
reap_agent_pids() directly to poc-hook.rs and call it on every
UserPromptSubmit. Kills timed-out agents (10min) and cleans up
pid files for dead processes.

Also remove dead subconscious/subconscious.rs (420 lines) — was
forked to claude/agent_cycles.rs and never removed.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/subconscious/mod.rs

@@ -24,4 +24,3 @@ pub mod defs;
 pub mod digest;
 pub mod learn;
 pub mod prompts;
-pub mod subconscious;