docs: route community feedback through discussions

[anusbutt]

Outcome

Route open-ended community conversation through GitHub Discussions while preserving Issues for reproducible bugs and bounded implementation work.

Changes

• link Discussions, categories, and the pinned welcome announcement from the README and support guidance
• add a Discussions contact route to the issue chooser
• document the Discussion-to-Issue-to-PR lifecycle for contributors
• record the enabled categories and moderation intent in repository settings
• extend documentation contract tests

Evidence

• GitHub Discussions enabled and verified
• categories verified: Announcements, Q&A, Ideas, Design discussions, Show and tell
• welcome discussion Welcome to GraphKeeper Discussions #6 verified as globally pinned
• git diff --check
• npm run test:integration: 123 passed, 0 failed

Compatibility and recovery

Documentation and repository community routing only. No runtime, schema, package identity, platform, or release behavior changes. Discussions can be disabled independently without changing GraphKeeper data.