upgrade-all: skip 'external' recipes (uptime-kuma) + add used-recipes.md inventory

Operator: uptime-kuma is maintained elsewhere — drop it from the weekly upgrade
but keep it in the used-recipes inventory. New cc-ci-plan/used-recipes.md is the
canonical list of every recipe cc-ci deploys/tests, with a weekly|external tier;
upgrade-all §1 now excludes 'external' rows from the candidate list (explicit
--args still override). uptime-kuma = external; all others weekly.

cc-ci-plan/used-recipes.md

@@ -0,0 +1,36 @@
+# Used recipes — the canonical inventory of every recipe cc-ci deploys/tests
+
+Format: `<recipe>  <tier>  [note]` — one row per recipe. Tier:
+
+- **`weekly`** — cc-ci maintains it: included in the weekly `/upgrade-all` survey **and** upgrade
+  (an upgrade PR is opened when a new upstream version exists).
+- **`external`** — cc-ci **uses/tests** it (it's deployed + has `tests/<recipe>/`), but **someone
+  else maintains/upgrades it**, so it is **excluded from the weekly upgrade** — no upgrade PR is
+  opened for it. It still runs in CI like any other recipe; we just don't version-bump it.
+
+`/upgrade-all` reads this file and upgrades only the `weekly` rows (see the skill §1). This list is
+the source of truth for "what recipes do we use" — keep it in sync when recipes are enrolled/retired.
+
+```
+bluesky-pds       weekly
+cryptpad          weekly
+custom-html       weekly
+custom-html-tiny  weekly
+discourse         weekly
+drone             weekly
+ghost             weekly
+gitea             weekly
+hedgedoc          weekly
+immich            weekly
+keycloak          weekly
+lasuite-docs      weekly
+lasuite-drive     weekly
+lasuite-meet      weekly
+mailu             weekly
+matrix-synapse    weekly
+mattermost-lts    weekly
+mumble            weekly
+n8n               weekly
+plausible         weekly
+uptime-kuma       external   # maintained elsewhere — used/tested by cc-ci but NOT weekly-upgraded
+```