[codex] Add public growth plan

[Schoaib]

Summary

• copy and adapt the agent-ready OSS growth plan into the public repo
• keep local automation details, private contacts, secrets, outreach trackers, recaps, and raw metrics out of the committed plan
• add targeted .gitignore rules for private launch artifacts without ignoring the whole launch folder

Validation

• git check-ignore verified private launch paths and .claude are ignored
• Not run: docs-only change