docs: add COLLABORATORS.md

[reinkrul]

Summary

• Adds COLLABORATORS.md describing how to contribute: when to open an issue first, keep PRs small, signed commits, Conventional Commits, matching surrounding code conventions, tests, codegen via the makefile, and review etiquette (no force-push after review starts).
• Points the existing contribution touchpoints at the new file instead of duplicating content:
  • README.rst — adds a "Contributing" section linking to it.
  • .github/README.md — adds a bullet in "For Contributors".
  • docs/pages/contribute.rst — slimmed to a pointer (was a one-liner already).

Test plan

• Render COLLABORATORS.md on GitHub and check links resolve.
• Render docs/pages/contribute.rst on Read the Docs and check the link resolves.

Assisted by AI

[qltysh]

Coverage Impact

⬇️ Merging this pull request will decrease total coverage on master by 0.01%.

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[stevenvegt]

also add that we cannot accept breaking changes. api, config, behavior etc.

[stevenvegt]

Is this also the place to add the LLM/AI policies?

[reinkrul]

also add that we cannot accept breaking changes. api, config, behavior etc.

I think we'll be repeating things that are already covered in other places. If we want to document workings of specific parts (we're using Koanf) other than what is already documented (env variables, cli flags, etc), I'd rather do that in markdown files next to the code, otherwise this is going to be another very big doc that's going to be out-of-date soon.

[stevenvegt]

I think we'll be repeating things that are already covered in other places.
If that is the case, can we link to that?

[reinkrul]

Added a "Backwards compatibility" section that states we don't accept breaking changes to public interfaces (APIs, config, behaviour, network protocols) and links to the existing docs rather than restating them:

• docs/pages/development/3-api.rst — API versioning rules.
• docs/pages/development/2-releasing.rst — semantic versioning, including the note that "API" is scoped broadly (config, network protocols).