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-tty-utils/src/lib.rs

@@ -40,6 +40,7 @@ use std::collections::HashMap;
 use std::io;
 
 mod process_scope;
+pub mod runtime;
 pub use process_scope::{ProcessScope, global_process_scope};
 
 // TTY detach — pre_exec building block