Add CONTRIBUTORS.md: issues-only policy (share prompts, not PRs)

[cliffhall]

Summary

Add a new CONTRIBUTORS.md at the repository root that documents our contribution policy going forward: we accept issues, not pull requests. Design and implementation are done by the maintainers. When a contributor has already built a fix or feature locally, they should share the prompt they used to produce it — not the source code.

Why

The Inspector v2 is developed with an AI-assisted, prompt-driven workflow with a consistent architecture, design system, and testing gates (see AGENTS.md). Accepting raw source PRs creates friction: they must be reverse-engineered to fit our component/theme conventions, coverage gates, and review process. Capturing the intent (a well-formed issue) or the prompt that generated a local change lets us reproduce the work within our own pipeline and standards.

Requirements

Create CONTRIBUTORS.md (root) covering:

• Policy statement: We accept issues only. Pull requests from external contributors will not be merged. The maintainers handle design and implementation.
• How to contribute a bug report or feature request: open a well-scoped issue on the appropriate version board (v2 work → v2 board / v2/main). Reference the labeling-by-version convention from AGENTS.md (v1 / v1.5 / v2).
• If you've already fixed it locally: don't send a diff/PR. Instead, share the prompt(s) you used to generate the change, plus a description of the behavior before/after and how you verified it. We'll reproduce it through our own workflow.
• What makes a good issue / prompt submission: clear reproduction or use case, expected vs. actual behavior, affected client (Web / TUI / CLI), and the exact prompt text if applicable.
• Rationale: briefly explain the prompt-driven workflow so the policy reads as intentional, not dismissive.
• Cross-reference: link from README.md (and/or AGENTS.md) to CONTRIBUTORS.md so it's discoverable. Update the docs per the repo's "Keep documentation files up to date" maintenance rule.

Notes

• Keep the tone welcoming — contributors are valued; the policy is about channeling their input into a form we can act on efficiently.
• Consider whether a GitHub PR template / CONTRIBUTING.md redirect is also warranted to steer would-be PR authors to this policy (optional follow-up).