refact(deployer): the LLM handlers gate through the shared library

deploy_llm_dragoon/zealot/arkon now call require_image / require_bytes /
require_min_mib / require_file / require_dir_files, so the deployer and the
serve scripts refuse the same missing payload with the same words — the
2026-10-01 crash-loop law, enforced by construction instead of by duplicated
shell. require_exec joins the library for the native-runtime gate (a
compiled tree that lost its +x refuses like a missing one). relink_unit
takes SYSTEMD_SYSTEM_DIR so a handler can be driven end to end in a sandbox.

The library's own refusals now carry their status: a bash function that
ends inside an if returns the CONDITION's status, so a gate whose last
statement is its refusal printed the message and returned SUCCESS — the
new seam caught it, and the gate tests pin it.

New seam tests/llm_deploy_gates.bash + tests/stubs/: deploy-remote.sh's LLM
handlers EXECUTED against stub systemctl/podman/sudo/ss/curl, asserting
exit status, refusal text, no restart, and the pending state marker.

tests/frank_dragoon_unit.bats

@@ -204,7 +204,7 @@ good_env() {
     run env PATH="$TEST_TMP/bin:$PATH" "${GOOD_ENV[@]}" \
         FRANK_DRAGOON_NGRAM_TABLE=absent.hgn "$SERVE"
     [ "$status" -ne 0 ]
-    [[ "$output" == *"n-gram-table"*"missing"* ]]
+    [[ "$output" == *"frank-dragoon: n-gram: missing at $TEST_TMP/absent.hgn"* ]]
     run env PATH="$TEST_TMP/bin:$PATH" "${GOOD_ENV[@]}" \
         FRANK_DRAGOON_NGRAM_BYTES=42 "$SERVE"
     [ "$status" -ne 0 ]
@@ -220,7 +220,7 @@ good_env() {
     run env PATH="$TEST_TMP/bin:$PATH" "${GOOD_ENV[@]}" \
         FRANK_DRAGOON_VISION_TOWER=absent.hgn "$SERVE"
     [ "$status" -ne 0 ]
-    [[ "$output" == *"vision-tower"*"missing"* ]]
+    [[ "$output" == *"frank-dragoon: vision: missing at $TEST_TMP/absent.hgn"* ]]
     run env PATH="$TEST_TMP/bin:$PATH" "${GOOD_ENV[@]}" \
         FRANK_DRAGOON_VISION_BYTES=42 "$SERVE"
     [ "$status" -ne 0 ]