Add GitHub workflow guidance

[austin-smith]

What Changed

• Added structured bug report and feature request forms.
• Added a pull request template with change, rationale, related issue, and validation prompts.
• Added branch, commit, pull request, and issue conventions to AGENTS.md.

Why

Align PlexBar's GitHub contribution workflow with BitDream while preserving PlexBar-specific platform, environment, and label guidance.

Validation

• Parsed all issue-template YAML files successfully.
• Ran git diff --cached --check successfully before committing.
• Application tests were not run because this changes only repository guidance and GitHub templates.