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- CHANGELOG.md: note under Unreleased (no tag bump / unpublished)

docker-compose.yml

@@ -31,9 +31,13 @@ services:
       - .env
     environment:
       - TERM=xterm-256color
-      - GITEA_ACCESS_TOKEN=${GITEA_ACCESS_TOKEN:-}
-      - GITEA_HOST=${GITEA_HOST:-}
-      - GITHUB_PERSONAL_ACCESS_TOKEN=${GITHUB_PERSONAL_ACCESS_TOKEN:-}
+      # Secrets (GITEA_*, GITHUB_*, and any others) are delivered to the
+      # container via `env_file: .env` above — do NOT duplicate them here.
+      # An `environment:` entry overrides env_file AND is interpolated from
+      # the host shell, so a stale shell export (e.g. one auto-loaded by a
+      # dotenv hook) would silently shadow the value in your .env. Keeping
+      # secrets env_file-only decouples the container from the host shell.
+      # See .env.example for the full list of supported variables.
     volumes:
       # Host workspace — mount your project here
       - ${WORKSPACE_PATH:-.}:/workspace