fix(agents): bound how many subagents one session runs at once

Port of upstream grok-build 1.0.1 (landed upstream 2026-08-06). The
coordinator fire-and-forgot every spawn, so a wide fan-out (graph
waves, swarm bursts, model-driven Task storms) could exhaust file
descriptors.

- kigi-tools task/admission: SubagentLimits — 32 concurrent per session
  by default (KIGI_MAX_CONCURRENT_SUBAGENTS), queue|fail behavior
  (KIGI_SUBAGENT_LIMIT_BEHAVIOR), zero clamped to 1, read once at the
  composition root; parse_positive_env helper.
- Each parent session gets one spawn semaphore (map entries live for
  the process so a resumed session shares its predecessor's limit);
  handle_subagent_request holds an owned permit for the whole child
  run — queue awaits FIFO with a unified-log notice at saturation,
  fail routes through send_pre_spawn_failure. After a queued wait, a
  closed parent command channel (teardown) fails the spawn instead of
  running against a dead parent.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KSy9S31pZNWXtRZNVJ1sDf

crates/codegen/kigi-shell/src/agent/subagent/mod.rs

@@ -133,6 +133,13 @@ impl AutoCompactThresholdTiers {
 pub(crate) struct SubagentSpawnContext {
     /// Parent's LSP runtime — inherited via ToolContext, same as fs/terminal.
     pub lsp: Option<std::sync::Arc<dyn kigi_tools::implementations::lsp::LspBackend>>,
+    /// Session-scoped spawn limits (env-resolved once at the composition root).
+    pub spawn_limits: kigi_tools::implementations::kigi::task::admission::SubagentLimits,
+    /// Parent session's spawn semaphore, sized to
+    /// `spawn_limits.effective_max_concurrent()`. A permit is held for the
+    /// whole child run, so a wide fan-out queues (or fails, per behavior)
+    /// instead of exhausting file descriptors.
+    pub spawn_permits: std::sync::Arc<tokio::sync::Semaphore>,
     #[expect(
         dead_code,
         reason = "unused in production; remove expect when wired or delete the item"