compose: deliver secrets via env_file only (drop environment: passthrough)

Removes GITEA_ACCESS_TOKEN / GITEA_HOST / GITHUB_PERSONAL_ACCESS_TOKEN from
the compose environment: block. An environment: entry both overrides
env_file AND is interpolated from the host shell, so a stale shell export
(e.g. one auto-loaded by an opencode/dotenv hook) silently shadowed the
users .env — an updated token never reached the container. Secrets now flow
solely via env_file: .env; .env.example already documents every variable.

- docker-compose.yml: drop the 3 passthrough lines + explanatory comment
- README.md: sync the "basic shape" snippet
- CHANGELOG.md: note under Unreleased (no tag bump / unpublished)

README.md

@@ -319,9 +319,10 @@ services:
     environment:
       - TERM=xterm-256color
       # - STUDIO_EXPOSE=1          # -studio only: auto-start the socat bridge on boot
-      - GITEA_ACCESS_TOKEN=${GITEA_ACCESS_TOKEN:-}
-      - GITEA_HOST=${GITEA_HOST:-}
-      - GITHUB_PERSONAL_ACCESS_TOKEN=${GITHUB_PERSONAL_ACCESS_TOKEN:-}
+      # Secrets (GITEA_*, GITHUB_*, …) come from env_file: .env above — not
+      # duplicated here. An environment: entry overrides env_file and is
+      # interpolated from the host shell, so a stale shell export would
+      # silently shadow your .env. See .env.example for the full list.
     volumes:
       # Workspace: your host source tree
       - ${WORKSPACE_PATH:-.}:/workspace