docs(readme): clarify branch model

[emmepra]

Summary

• clarify that main is the stable public branch
• state that releases should be cut from main
• position dev as the integration branch for ongoing work

Scope

• in scope:
  • README branch policy wording
• out of scope:
  • code or runtime changes
  • release automation changes

Validation

• relevant tests/checks were run
• Docker/runtime validation was run when the change touched backend, orchestration, templates, or integration paths
• docs/rules were updated when behavior, contracts, or workflow changed

Commands run:

git diff -- README.md

Risks

• user-visible regressions: low; docs-only change
• operational/runtime risks: none expected
• follow-up work:
  • promote dev into main
  • switch the GitHub default branch to main

Evidence

• logs: README diff review
• screenshots: none
• related issues: none