fix(gitlab-runner-vm): close remaining review gaps in executor and provisioning

[waynesun09]

Thanks for taking the first round into a18c3db4 — this is now just the follow-up delta from a Grok review of that fix commit, rebased onto your current head. Your PRE_COMMIT_HOME/GOCACHE envs and the chmod 700 are preserved. One commit, +122/−42.

What this fixes

Executor

• --env NAME instead of --env NAME=VALUE — podman copies the value from its own environment, so job variables (including file-type PEM material and CI_JOB_TOKEN) no longer appear in podman's argv / /proc/<pid>/cmdline. Exports happen in a subshell that execs podman, so a job variable named PATH or HOME can't alter run.sh; the handful of names that steer the podman process itself (PATH, HOME, TMPDIR, XDG_*, CONTAINERS_*, LD_*) go inline since they're never secrets.
• cleanup.sh no longer returns early on an unexpected container name — the staged copy of the gateway mTLS material is removed regardless.
• Control characters rejected in resolved bind-mount paths; real reason reported when a stale container can't be removed.
• Fixture test grows to 21 cases with job-identity coverage: spoofed CUSTOM_ENV_CI_JOB_ID is ignored; missing / unreadable / non-int / malformed JOB_RESPONSE_FILE all fail before podman.

Provisioning

• Remote trap fix (the one HIGH): the previous trap 'rm -f .env' EXIT INT TERM HUP handler didn't exit, so it swallowed the SIGHUP from a dropped SSH connection and let setup.sh keep running while the local side deregistered the runner. Now .env write + setup.sh run in one remote session with an EXIT trap plus HUP/INT/TERM handlers that terminate the shell — and there's no longer a window where the token file exists without a trap covering it.
• INT/TERM rollback traps disarmed on success (a stray Ctrl-C during the final "Done." output was deregistering a healthy runner); TERM exits 143.
• VM_USER templated into cloud-init (__VM_USER__ in vm.yaml — the only vm.yaml change; resources untouched) so the knob is wired end to end; VM_USER and RUNNER_ACCESS_LEVEL validated before the VM exists.
• TOFU chain: every certificate validated (openssl crl2pkcs7 | pkcs7 -print_certs), not just the leaf, so a mid-chain truncation can't install.
• Active-gateway marker matched with optional leading whitespace; (NotFound) matched exactly in delete-vm.sh.

Verification

• bash -n + shellcheck -x -e SC1091,SC2001,SC2016 clean on all scripts
• Full pre-commit hook set passes (incl. gitlint)
• Fixture test 21/21
• 0 behind fullsend-ai/fullsend:main, git merge-tree clean