chore: add contribution workflow - #6

What changed

• adds focused contribution guidance for the develop-to-release branch model;
• adds structured bug and feature forms and a pull request template;
• commits the Cargo lockfile for reproducible native dependency resolution;
• links the README to the contribution requirements.

Why

Planer already has a strong README, security policy, code of conduct, and Windows release pipeline. This change fills the remaining contribution and issue-intake gaps without claiming tests that do not yet exist.

Validation

• YAML parsing for every issue form and workflow
• npm ci
• npm run check:version
• npm run lint
• npm run build
• cargo check --manifest-path src-tauri/Cargo.toml

Known baseline

npm audit reports transitive advisories in the Excalidraw/Mermaid dependency graph. The suggested automatic fix downgrades the direct Excalidraw dependency and is not safe to combine with this repository-governance change.