Spacebar toggle for all agents, persist to config, scan agent directory

- Scan agents directory for all .agent files instead of hardcoded list
- Persist enabled state to ~/.consciousness/agent-enabled.json
- Spacebar on F3 agent list toggles selected agent on/off
- Both subconscious and unconscious agents support toggle
- Disabled agents shown dimmed with "off" indicator
- New agents default to disabled (safe default)

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

src/subconscious/defs.rs

@@ -163,7 +163,7 @@ pub fn agents_dir() -> PathBuf {
 }
 
 /// Load all agent definitions.
-fn load_defs() -> Vec<AgentDef> {
+pub fn load_defs() -> Vec<AgentDef> {
     let dir = agents_dir();
     let Ok(entries) = std::fs::read_dir(&dir) else { return Vec::new() };