fix(startup): thread-exhaustion resilience — cap workers, never panic

Port of upstream grok-build 0.2.113/0.2.114 fixes, extended to Kigi's
startup order.

- kigi-tty-utils runtime module: worker-cap policy (8 max) — tokio's
  per-core default pins 100+ thread slots on many-core shared hosts
  against RLIMIT_NPROC / pids.max, and later spawns die with EAGAIN.
- kigi-bin builds the main runtime with capped workers and reports an
  out-of-threads host with a diagnostic + exit(1) instead of a panic.
- The pre-runtime optimization threads (HTTP warmup, early model
  prefetch) spawn recoverably and degrade — their panic would have
  preempted the runtime's own diagnostic.
- The per-session actor thread and its runtime report spawn/build
  failure through the existing acp::Error / AgentBuildError boundaries
  (new RuntimeBuild variant) instead of panicking the agent host.
- fsnotify's watcher thread spawn maps into FsNotifyError::WatcherStart
  so callers degrade instead of crashing.

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

crates/codegen/kigi-agent/src/error.rs

@@ -17,6 +17,11 @@ pub enum AgentBuildError {
     #[error("IO error during agent construction: {0}")]
     IoError(#[from] std::io::Error),
 
+    /// The per-session runtime could not be built — fd or thread
+    /// exhaustion (EMFILE/EAGAIN), not a definition problem.
+    #[error("failed to build the session runtime: {0}")]
+    RuntimeBuild(std::io::Error),
+
     /// Carries template line numbers and surrounding context.
     #[error("template rendering error: {0}")]
     MiniJinjaError(#[from] minijinja::Error),