ci: strengthen contribution governance

[limityan]

Summary

Add lightweight contribution governance for BitFun:

• Default PR template with verification evidence fields instead of duplicated command lists.
• Short issue forms for bugs, product proposals, and documentation issues.
• Repository hygiene and GitHub YAML validation scripts.
• CI coverage for web UI tests and mobile-web type-check/build.
• Updated AGENTS/CONTRIBUTING guidance for mobile-web and focused verification.

Motivation / Product Context

As BitFun accepts broader open-source contributions across desktop, web UI, mobile web, relay, and agent runtime surfaces, maintainers need enough context to triage and review without making contribution templates feel like long forms.

Fixes #N/A

Type and Areas

Type: Docs / CI / repository governance

Areas: GitHub templates, CI, AGENTS, CONTRIBUTING, mobile web guidance

User Impact

No direct runtime user-facing behavior changes. Contributors get clearer and lighter issue/PR intake, while maintainers get stronger automated checks for template/config validity and frontend coverage.

Design / Architecture Notes

The command selection matrix lives in CONTRIBUTING.md as the source of truth. The PR template records verification evidence instead of duplicating command lists. Issue templates keep only the minimum fields needed for triage.

Verification

Verification scope:

Repository metadata, GitHub templates, GitHub workflow, and docs guidance.

Commands run and results:

• pnpm run check:repo-hygiene -> pass
• pnpm run check:github-config -> pass
• git diff --check -> pass
• git diff --cached --check -> pass
• pnpm --dir src/web-ui run test:run -> pass, 145 files / 768 tests
• pnpm run lint:web -> pass
• pnpm run build:web -> pass
• pnpm --dir src/mobile-web run type-check -> pass
• pnpm run build:mobile-web -> pass

Manual verification:

N/A. This PR changes repository governance, CI, and contribution documentation only.

Skipped checks:

Rust and desktop runtime checks were not rerun for the latest template-only refinement because no Rust, Tauri, desktop runtime, or product behavior code changed.

Risk and Rollback

Main risk is modest CI duration increase from web UI tests and mobile-web checks. Current GitHub Actions measurement showed Frontend Build increasing from about 1m51s to about 2m36s, while the full workflow remains dominated by the Rust Windows job. Rollback is straightforward by reverting this PR.

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, or skipped checks are explained.
• User-facing strings, docs, and AI-assisted work disclosure are handled where applicable.