docs(contributing): apply issue-first policy to beta

[ifBars]

Bug fixes, features, and API improvements now require a GitHub issue opened before the PR, with duplicate searches and an intended approach recorded on the issue. The PR template requests the issue link and contributor checklist; issue forms, README, and agent guidance reinforce the same policy.

Minor typo, formatting, and documentation-only changes retain a documented exception. GitHub issues replace the stale Trello tracking guidance, and contributor branch targets now match stable/beta policy.

Validation: issue-form YAML parses successfully; git diff --check passes. No runtime code or CI workflows changed. This documentation-only policy change uses the documented issue exception.

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