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/agent.rs

@@ -561,6 +561,10 @@ impl Agent {
         self.config.max_tool_result_chars
     }
 
+    pub fn max_concurrent_jobs(&self) -> Option<usize> {
+        self.config.max_concurrent_jobs
+    }
+
     pub fn compression_keep_last(&self) -> Option<usize> {
         self.config.compression_keep_last
     }
@@ -735,6 +739,8 @@ pub struct AgentConfig {
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub max_tool_result_chars: Option<usize>,
     #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub max_concurrent_jobs: Option<usize>,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
     pub compression_keep_last: Option<usize>,
     #[serde(default)]
     pub description: String,