v0.2.0

refactor: rename PingMailer/Silver to OpenGovMail (#37)

The project is transferring to the OpenGovMail org and consolidating on a single
name, so every Silver-Mail-Platform, LSFLK and Silver reference is renamed to
OpenGovMail / opengovmail.

- Go module: github.com/Silver-Mail-Platform/pingmailer -> github.com/OpenGovMail/opengovmail
- Images: ghcr.io/{silver-mail-platform,lsflk}/* -> ghcr.io/opengovmail/*, with
  pingmailer-api-server, silver-dkim, silver-smtp and silver-certbot renamed to
  opengovmail-{api-server,dkim,smtp,certbot}
- Helm: umbrella chart `pingmailer` -> `opengovmail`; template helper prefixes
  silver-* -> opengovmail-*; chart maintainers and sources point at the new org
- Paths: mail-infra/{conf/silver.yaml,services/silver-config,images/silver-*} and
  scripts/service/{start,stop}-silver.sh renamed to their opengovmail equivalents
- Env vars: SILVER_YAML_FILE -> OPENGOVMAIL_YAML_FILE, SILVER_CONFIG -> OPENGOVMAIL_CONFIG
- Docs, NOTES.txt, issue template and workflow comments updated to match

Verified: `go build ./...` and `go vet ./...` clean, `helm lint` and
`helm template` succeed for the umbrella and certbot charts, `bash -n` passes on
every shell script, and no case-insensitive match for silver/lsflk/pingmail
remains in the tree.