feat: add PRD template for scoping larger features

From the PR description

Summary

• Adds a lightweight PRD template (docs/templates/PRD.md) for scoping larger features/architecture changes, covering Problem, Non-goals/out of scope, Proposed solution, Alternatives considered, Technical approach/affected areas, Success metrics, and Open questions.
• Adds a matching GitHub issue form (.github/ISSUE_TEMPLATE/prd.md) so "New PRD" appears as an issue-creation option, using the same sections.
• Adds one sentence to CONTRIBUTING.md on when to use the PRD template vs. a plain issue.

Closes #315

Changes

• docs/templates/PRD.md - new PRD template, copy-paste source for scoping issues.
• .github/ISSUE_TEMPLATE/prd.md - new issue form with YAML frontmatter (name, about, title) and the same section headers as the template.
• CONTRIBUTING.md - one-line guideline on when to reach for the PRD template vs. a plain issue.
• .gitignore - ignore dev-cycle scratch files (.current-issue.md, .implementation-plan.md, .implement-config).

Test plan

• No application code touched - docs/template-only change.
• Verified .github/ISSUE_TEMPLATE/prd.md YAML frontmatter parses correctly (Python yaml.safe_load), since the repo has no existing pattern for validating issue-template YAML.
• npm run build --prefix backend passes.
• npm run lint --prefix frontend passes (0 errors).
• npm run build --prefix frontend passes (includes typecheck), using CI's placeholder NEXT_PUBLIC_* env vars.