docs: add CONTRIBUTORS.md issues-only policy (#1517)

[cliffhall]

Closes #1517.

Summary

Adds a root CONTRIBUTORS.md documenting the contribution policy: we accept issues, not pull requests. Maintainers handle design and implementation through the prompt-driven workflow described in AGENTS.md. Contributors who have already built a change locally are asked to share the prompt they used, not a diff.

The doc covers every requirement from the issue:

• Policy statement — issues only; external PRs are not merged; maintainers own design/implementation.
• How to report a bug / request a feature — open a well-scoped issue on the right version board, with a table mapping v2 / v1.5 / v1 to their boards, base branches, and labels (cross-referencing the "Label by version" convention in AGENTS.md).
• If you've already fixed it locally — don't send a diff; share the prompt(s), before/after behavior, and how you verified.
• What makes a good issue / prompt submission — reproduction or use case, expected vs. actual, affected client (Web / TUI / CLI), and exact prompt text.
• Rationale — explains the prompt-driven workflow so the policy reads as intentional, with a welcoming tone.
• Cross-reference — adds a "Contributing" section to AGENTS.md linking to CONTRIBUTORS.md. (There is no root README.md in this repo, so AGENTS.md is the cross-reference point — the issue allows "README.md and/or AGENTS.md".)

Notes

• Docs-only change to root files; no client code is touched, so the build/test gates don't apply (no root-level prettier scope covers these files either).
• The optional follow-up (a CONTRIBUTING.md redirect / PR template to steer would-be PR authors) is left for a separate issue if desired.

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