docs: add issue-first contribution policy

[alessandrobologna]

Summary

• Add a root contribution guide that documents Khone's issue-first contribution model.
• Add structured GitHub issue forms for reproducible bug reports, feature requests, and design/use-case feedback.
• Disable blank issues so reports are routed through the evidence-focused templates.
• Link the contribution policy from the README.

Rationale

This documents the project's boundary around unsolicited external pull requests while keeping GitHub issues open for practical feedback, substantiated bugs, feature requests, and real-world use cases. The policy names the maintainer-capacity, project-direction, and CI/CD supply-chain concerns behind that boundary.

Closes #13

Verification

• git diff --check
• ruby -e 'require "yaml"; ARGV.each { |f| YAML.load_file(f); puts "ok #{f}" }' .github/ISSUE_TEMPLATE/*.yml

[Copilot]

Pull request overview

Adds contributor-facing documentation and GitHub issue intake structure to enforce an “issue-first” contribution model for Khone, routing external feedback through evidence-focused issue templates instead of unsolicited PRs.

Changes:

• Added a root CONTRIBUTING.md explaining the issue-first policy and rationale, plus guidance for bug/feature/security reports.
• Added GitHub Issue Forms for bug reports, feature requests, and design/use-case feedback, and disabled blank issues.
• Linked the contribution policy from the root README.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
README.md	Links contributors to the new contribution policy and clarifies PR boundary.
CONTRIBUTING.md	Documents the issue-first model, rationale, and reporting expectations (bug/feature/security).
.github/ISSUE_TEMPLATE/bug_report.yml	Adds a structured bug report form emphasizing reproducibility and evidence.
.github/ISSUE_TEMPLATE/feature_request.yml	Adds a feature request form focused on problem/use-case and constraints.
.github/ISSUE_TEMPLATE/design_feedback.yml	Adds a design/use-case feedback form to capture operational and API feedback.
.github/ISSUE_TEMPLATE/config.yml	Disables blank issues to route reports through templates.

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