Fix adapter signal handling on Linux

[yihanzhu]

Summary

• Keep one permanent guarded signal handler instead of swapping traps around process launch.
• Defer signals only across child PID/process-group registration, then replay them.
• Make every trap-reachable state read safe under set -u, including repeated TERM.
• Prevent signal cleanup from re-entering.

Why

PR #206 exposed a Linux-only failure in the already-merged #205 signal regression: the runner exited with Bash status 2 and leaked stderr even though the child group and scratch state were cleaned. This one-file follow-up fixes that root cause before #206 continues.

Proof

Exact head/base: 9c79b78ce98987ac50e050abf563fc877c541943 / cac3a2eaf41000c14d84b6c4af64b09fbdab7ce5.

• adapter contract matrix: 29/29 locally, including TERM group cleanup
• ShellCheck 0.11.0: clean
• Bash syntax and diff checks: clean
• Linux proof: required CI for this exact head

Inactive fake-only test runtime. No real adapter, credential, network use, activation, release, or deployment.

Follow-up to #205; unblocks #206.

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[yihanzhu]

Codex reviewer (cross-vendor, read-only)

Reviewed-head: 9c79b78
Reviewed-base: cac3a2e
reviewer: operator-default @ high

Posted verbatim by codex-review.sh (codex exec review --json --base refs/codex-review/207-29867/base in an isolated temp worktree, sandbox forced read-only). Comments only — Codex never pushes, approves, or merges.

The permanent signal handler closes the handler-replacement race while preserving deferred delivery during child process-group setup. The nounset-safe defaults and cleanup guard do not alter normal behavior, and the changed script passes syntax and pinned ShellCheck validation.