Polish README and env examples for OSS launch

[mithileshchellappan]

Summary

• Rewrite the README for open-source release with a contents page, clearer positioning, architecture overview, Live Activity coverage, and a direct comparison table against FCM, OneSignal, AWS SNS, and Gorush
• Update .env.example to match the current server/runtime configuration and remove obsolete SQLite/Firebase placeholders
• Remove the learnings/ folder from the published repo and add the standard OSS repo files, Docker setup, CI, OpenAPI spec, and license

Testing

• Ran OpenAPI and YAML validation
• Ran git diff --check and ASCII checks
• Ran go test ./...
• Verified the Docker image and compose stack by building and starting the service locally

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Fast-forward existing installs to fetched version

In scripts/setup.sh, the existing-repo path fetches origin "$VERSION" and then runs git checkout "$VERSION", but this does not advance a local branch (for example main) to the newly fetched commit. In practice, rerunning the installer in an existing ~/pushboy checkout can leave users on stale code even though fetch succeeds, because only remote-tracking refs/FETCH_HEAD were updated. This breaks the script’s update behavior for the most common upgrade flow.

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