🚀 Enable shared act_runner cache-server 🚀

Turned on the act_runner Actions cache instead of leaving it disabled.
The standalone `cache-server` process was already being launched
unconditionally by zz-act_runner.sh but had no `external_secret`, so it
was refusing to start (the binary requires a non-empty secret even
though its own `cache.enabled` field is unused). Wired a shared,
randomly generated secret through the existing REPLACE_* templating
pattern so the "gitea" runner, the cache-server, and the extra
runner-N daemons spawned by start-runners all share one cache backend.

- rootfs/tmp/etc/act_runner/default_config.yaml: cache.enabled: true;
  added external_server (pointed at the local cache-server) and
  external_secret placeholders
- rootfs/tmp/etc/act_runner/cache_server.yaml: cache.enabled: true;
  added external_secret placeholder (required for the binary to start)
- rootfs/usr/local/etc/docker/init.d/zz-act_runner.sh: added
  __gen_cache_secret() (mirrors __gen_auth_token's persisted-token
  pattern, stored at $CONF_DIR/tokens/cache_secret); substitutes
  REPLACE_RUNNER_CACHE_SECRET into both the runner config and the
  cache-server config; exports RUNNER_CACHE_HOST/PORT/SECRET for
  start-runners; bumped version stamp
- rootfs/usr/local/bin/start-runners: generates a shared
  runners-cache.yaml from the exported cache env vars and passes
  --config to both `act_runner register` and `act_runner daemon` for
  every runner-N instance, so they use the same external cache server
  instead of an unshared per-process local cache
- TODO.AI.md: logged pre-existing script-lint findings (missing `--`
  before grep queries throughout zz-act_runner.sh, a UUOC in
  start-runners, an unpinned/stale docker.yaml CI workflow, and
  forbidden OCI labels in the Dockerfile) surfaced incidentally by the
  lint pass for this change but out of scope for it

rootfs/tmp/etc/act_runner/cache_server.yaml

@@ -2,7 +2,8 @@ log:
   level: warn
 
 cache:
-  enabled: false
+  enabled: true
   dir: 'REPLACE_RUNNER_CACHE_DIR'
   host: '0.0.0.0'
   port: REPLACE_RUNNER_CACHE_PORT
+  external_secret: 'REPLACE_RUNNER_CACHE_SECRET'

rootfs/tmp/etc/act_runner/default_config.yaml

@@ -66,4 +66,6 @@ container:
   docker_host: ''
 
 cache:
-  enabled: false
+  enabled: true
+  external_server: 'http://REPLACE_RUNNER_CACHE_HOST:REPLACE_RUNNER_CACHE_PORT/'
+  external_secret: 'REPLACE_RUNNER_CACHE_SECRET'