refactor(ai-roles): make catalog URL a per-branch image default, drop local-fs source

The agent-roles catalog source is no longer hardcoded in app code and no longer
supports a local filesystem directory. The provider fetches only from an
http(s):// base URL read at runtime from AI_AGENT_ROLES_CATALOG_URL; an empty or
non-http value yields a 502 (catalog unavailable). The image ships a per-branch
default for that URL (set in CI), still overridable at runtime via the env var.

- provider: drop readLocal + node:fs/node:path; readRelative requires http(s)
  and 502s otherwise; remote fetch/streaming-cap/SSRF guards unchanged.
- environment.service: keep AI_AGENT_ROLES_CATALOG_URL (default ''); comment
  reflects the per-branch build-time default that is runtime-overridable.
- Dockerfile: add ARG+ENV AI_AGENT_ROLES_CATALOG_URL in the installer stage as
  the image default.
- CI: develop.yml builds with the develop raw URL; release.yml defines the main
  raw URL once in workflow env and references it from both build steps.
- tests: replace local-fixture tests with remote-mock happy/malformed bundle
  tests and a non-http => 502 case; path-traversal block uses an https source.
- docs: update .env.example, CHANGELOG (#222), agent-roles-catalog/README.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

CHANGELOG.md

@@ -39,8 +39,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   `/update-from-catalog` — and a new `source` column linking a role to its
   catalog slug/language/version. The catalog source is configured via the
   `AI_AGENT_ROLES_CATALOG_URL` env var — an `http(s)://` base URL to the
-  catalog's raw files, baked into the image at build time and set per branch in
-  CI (see `.env.example`). (#222)
+  catalog's raw files; the image ships a per-branch default baked in CI, and it
+  can be overridden at runtime via the env var (see `.env.example`). (#222)
 
 ### Fixed