fix: make lock acquisition signal-safe

[BartoszBlizniak]

What changed

• make lock acquisition and ownership recording indivisible to INT/TERM cleanup
• restore the installer signal handlers immediately after each acquisition attempt
• add a deterministic regression test that signals the installer immediately after mkdir creates the lock

Why

A signal arriving after mkdir succeeded but before LOCK_DIR was assigned could leave a permanent stale lock. Later installations would wait for 120 seconds and fail until the directory was removed manually.

Impact

Interrupted acquisition can no longer strand the installation lock, while waiting installers still cannot remove a lock owned by another process.

Validation

• shellcheck --shell=sh --severity=style install.sh
• bats tests/ (43 tests passed)
• git diff --check

[Copilot]

Pull request overview

• Purpose: harden install.sh lock acquisition against INT/TERM arriving between successful mkdir of the lock directory and recording ownership for cleanup, and add a regression test to prevent stale lock regressions.

Changes:

• Add enable_signal_traps() helper and use it from main() to centralize INT/TERM handling.
• Make lock acquisition + LOCK_DIR assignment effectively indivisible with temporary signal handling changes around mkdir.
• Add a deterministic Bats regression test that injects a TERM immediately after the lock directory is created.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
install.sh	Adjusts signal handling during lock acquisition so cleanup can’t strand a lock directory.
tests/install.bats	Adds a regression test that simulates a signal arriving immediately after lock creation.

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