feat: generalize supervisor registry to TaskHandle enum with job scaffolding, kill discipline, and max_concurrent_jobs config

Implements T1 of plans/background-jobs-design.md (§6, R7/R8/R9):

- Supervisor.handles is now HashMap<String, TaskHandle> where
  TaskHandle = Agent(AgentHandle) | Job(JobHandle); agent-facing
  accessors (active_count, effective_active_count, is_finished, take,
  inbox, abort_signal_for, list_agents) match only Agent variants,
  preserving all existing external behavior byte-for-byte.
- New JobHandle/JobState/JobStatus/JobResult types with pgid-guarded
  process-group kill discipline: Drop and cancel_all/cancel_recursive
  kill the group only while state.pgid is still set (pid-reuse guard),
  via libc::killpg on unix and JoinHandle::abort elsewhere.
- Per-kind job capacity: Supervisor carries max_concurrent_jobs
  (builder-set, default 0); job registration rejects at capacity.
- Cross-kind teaching errors at the four agent-lookup miss sites
  (agent__check/collect/cancel/send_message) when the id is a
  registered job or job_-prefixed; genuinely-unknown ids keep their
  existing messages.
- Supervisor init condition is now can_spawn_agents || jobs_enabled in
  use_agent and both child-agent spawn paths, with agent capacity 0 in
  jobs-only contexts; use_agent cancels the old supervisor recursively
  before replacing it.
- max_concurrent_jobs config plumbing: global Config field, AgentConfig
  override + accessor, all four AppConfig touch points including the
  COYOTE_MAX_CONCURRENT_JOBS env override; shared
  effective_max_concurrent_jobs/jobs_enabled predicates
  (agent override -> global -> default 5; 0 disables).
- Stage dependency-free RingBuf (64 KiB default) in src/function/jobs.rs
  for the upcoming job output pump.
- New sanctioned dependency: libc 0.2 under cfg(unix).

src/config/app_config.rs

@@ -68,6 +68,7 @@ pub struct AppConfig {
     pub summarization_prompt: Option<String>,
     pub summary_context_prompt: Option<String>,
     pub max_tool_result_chars: Option<usize>,
+    pub max_concurrent_jobs: Option<usize>,
 
     pub memory: Option<bool>,
     pub memory_cap_with_tools: Option<usize>,
@@ -153,6 +154,7 @@ impl Default for AppConfig {
             summarization_prompt: None,
             summary_context_prompt: None,
             max_tool_result_chars: None,
+            max_concurrent_jobs: None,
 
             memory: None,
             memory_cap_with_tools: None,
@@ -239,6 +241,7 @@ impl AppConfig {
             summarization_prompt: config.summarization_prompt,
             summary_context_prompt: config.summary_context_prompt,
             max_tool_result_chars: config.max_tool_result_chars,
+            max_concurrent_jobs: config.max_concurrent_jobs,
 
             memory: config.memory,
             memory_cap_with_tools: config.memory_cap_with_tools,
@@ -574,6 +577,9 @@ impl AppConfig {
         {
             self.compression_threshold = v;
         }
+        if let Some(v) = super::read_env_value::<usize>(&get_env_name("max_concurrent_jobs")) {
+            self.max_concurrent_jobs = v;
+        }
         if let Some(v) = super::read_env_value::<String>(&get_env_name("summarization_prompt")) {
             self.summarization_prompt = v;
         }
@@ -844,6 +850,49 @@ mod tests {
         }
     }
 
+    #[test]
+    fn from_config_copies_max_concurrent_jobs() {
+        let cfg = Config {
+            model_id: "test-model".to_string(),
+            max_concurrent_jobs: Some(3),
+            clients: vec![ClientConfig::default()],
+            ..Config::default()
+        };
+
+        let app = AppConfig::from_config(cfg).unwrap();
+
+        assert_eq!(app.max_concurrent_jobs, Some(3));
+    }
+
+    #[test]
+    #[serial_test::serial]
+    fn load_envs_overrides_max_concurrent_jobs() {
+        let env_name = get_env_name("max_concurrent_jobs");
+        let prev = std::env::var_os(&env_name);
+
+        let mut app = AppConfig::default();
+
+        unsafe { std::env::set_var(&env_name, "7") };
+        app.load_envs();
+        assert_eq!(app.max_concurrent_jobs, Some(7));
+
+        unsafe { std::env::set_var(&env_name, "0") };
+        app.load_envs();
+        assert_eq!(app.max_concurrent_jobs, Some(0));
+
+        unsafe { std::env::remove_var(&env_name) };
+        app.max_concurrent_jobs = Some(2);
+        app.load_envs();
+        assert_eq!(app.max_concurrent_jobs, Some(2));
+
+        unsafe {
+            match prev {
+                Some(v) => std::env::set_var(&env_name, v),
+                None => std::env::remove_var(&env_name),
+            }
+        }
+    }
+
     #[test]
     fn editor_returns_configured_value() {
         let configured = cached_editor()