fix(exec): bounded post-kill reaps — D-state children can't wedge a turn

Port of upstream grok-build 1.0.1 (landed upstream 2026-08-06).

A child wedged in an uninterruptible kernel syscall (D-state) never
observes SIGKILL, and a background descendant can hold the output pipes
open past the kill; both previously stalled tool futures without bound.

- kigi-tty-utils: KILL_REAP_TIMEOUT (5s) + reap_killed_bounded — wait
  for an already-killed child at most the bound, then abandon the corpse
  to tokio's orphan reaper.
- local_terminal: pipe readers share their buffer with the spawner and
  join under the reap bound on both exit arms (aborted readers are
  awaited so no tail bytes race the take); the whole tree is enrolled in
  a ProcessGroup and group-killed on timeout; the timeout arm returns
  the synthetic result instead of waiting on the corpse. Grandchild
  kill + prompt-return pinned by a ported unix regression test.
- kigi-tools: reap_killed_search_child wrapper; grep's four kill sites
  reap bounded; glob's stderr goes to null (an unread pipe blocks rg on
  warning backpressure) and its truncation kill reaps bounded.

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

crates/codegen/kigi-tty-utils/Cargo.toml

@@ -6,7 +6,7 @@ license = "Apache-2.0"
 description = "Lightweight process-spawning utilities for TTY safety — detach from controlling terminal, suppress interactive pagers, process-group lifecycle"
 
 [dependencies]
-tokio = { workspace = true, features = ["process"] }
+tokio = { workspace = true, features = ["process", "time"] }
 
 [target.'cfg(unix)'.dependencies]
 nix = { workspace = true, features = ["signal", "process"] }