feat(grafana): optional .grafana.env via GRAFANA_ENV_FILE, overridable GF_GITHUB_PLUGINS

[i-love-to-code]

This PR adds support for an optional Grafana configuration via an additional env file and makes the Grafana plugin list overridable from .env, without changing default behaviour for existing setups.

Changes

• Optional Grafana env file: Grafana service uses env_file: ${GRAFANA_ENV_FILE:-/dev/null}.
  • Unset or omit GRAFANA_ENV_FILE and behaviour is unchanged (no extra file).
  • Set GRAFANA_ENV_FILE=.grafana.env in .env and create .grafana.env from .grafana.env.tmp to pass extra GF_* options to Grafana.
  • .grafana.env is in .gitignore.
• Overridable GF_GITHUB_PLUGINS: In both docker-compose.yml and docker-compose.macos.yml, GF_GITHUB_PLUGINS is set via ${GF_GITHUB_PLUGINS:-"…"} so it can be overridden in .env while keeping the same default plugin URL.
• Docs: README documents the optional Grafana customisation (copy .grafana.env.tmp → .grafana.env, set GRAFANA_ENV_FILE=.grafana.env in .env). Sample .env in README and .env.tmp include commented GRAFANA_ENV_FILE and (where relevant) GF_GITHUB_PLUGINS. Fixed typos and Grafana docs link text.

Backwards compatibility

• No new required env vars or files.
• Default GRAFANA_ENV_FILE is /dev/null, so no file is required.
• GF_GITHUB_PLUGINS default is unchanged; only overrides are new.
• All existing environment vars remain and take precedence over the optional env file.