chore: add OSS community health files

Add standard open-source community files to make the project
welcoming and contribution-ready:

- CONTRIBUTING.md: dev setup, branching strategy, conventional
  commits, PR process, coding standards, and testing guidelines
- CODE_OF_CONDUCT.md: Contributor Covenant v2.0, contact updated
- SECURITY.md: responsible disclosure process with maintainer email
- .github/ISSUE_TEMPLATE/bug_report.md: structured bug reports
  with environment, config, and reproduction steps sections
- .github/ISSUE_TEMPLATE/feature_request.md: feature proposals
  with use-case and contribution-willingness fields
- .github/PULL_REQUEST_TEMPLATE.md: PR checklist aligned with
  the project's pre-commit and CI requirements

Author: luongnv89

.github/ISSUE_TEMPLATE/bug_report.md

@@ -0,0 +1,49 @@
+---
+name: Bug Report
+about: Report a bug to help us improve music-cli
+title: '[BUG] '
+labels: bug
+assignees: ''
+---
+
+## Bug Description
+
+A clear and concise description of the bug.
+
+## Steps to Reproduce
+
+1. Run `music-cli ...`
+2. ...
+3. See error
+
+## Expected Behavior
+
+What you expected to happen.
+
+## Actual Behavior
+
+What actually happened (include full error output / stack trace if available).
+
+## Environment
+
+- **OS:** [e.g., macOS 14.4, Ubuntu 22.04, Windows 11]
+- **music-cli version:** [run `music-cli --version`]
+- **Python version:** [run `python --version`]
+- **FFmpeg version:** [run `ffmpeg -version | head -1`]
+- **Install method:** [pip / uv / source]
+- **Optional extras installed:** [ai / youtube / none]
+
+## Configuration
+
+<details>
+<summary>~/.config/music-cli/config.toml (redact any personal info)</summary>
+
+```toml
+# paste here
+```
+
+</details>
+
+## Additional Context
+
+Add any other context, logs, or screenshots here.

.github/ISSUE_TEMPLATE/feature_request.md

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+---
+name: Feature Request
+about: Suggest an idea or improvement for music-cli
+title: '[FEATURE] '
+labels: enhancement
+assignees: ''
+---
+
+## Problem Statement
+
+A clear description of the problem or gap this feature would solve.
+*e.g. "I'm a developer who uses focus mode, but I can't quickly switch stations without stopping playback."*
+
+## Proposed Solution
+
+A clear description of what you want to happen.
+
+## Alternatives Considered
+
+Any alternative solutions or workarounds you've thought about.
+
+## Use Cases
+
+Describe specific scenarios where this would be valuable:
+
+1. ...
+2. ...
+
+## Would you be willing to contribute this?
+
+- [ ] Yes, I'd like to open a PR for this
+- [ ] I can help test / review
+- [ ] No, just suggesting
+
+## Additional Context
+
+Any mockups, references to similar tools, or extra context.

.github/PULL_REQUEST_TEMPLATE.md

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+## Description
+
+Brief description of what this PR changes and why.
+
+## Related Issue
+
+Fixes #<!-- issue number -->
+
+## Type of Change
+
+- [ ] Bug fix (non-breaking)
+- [ ] New feature (non-breaking)
+- [ ] Breaking change
+- [ ] Documentation update
+- [ ] Refactoring (no functional change)
+- [ ] CI / tooling / chore
+
+## Checklist
+
+- [ ] Code follows the project style guidelines (`pre-commit run --all-files` passes)
+- [ ] Self-reviewed the diff
+- [ ] Tests added / updated for changed behaviour
+- [ ] Documentation updated if behaviour changed (README, docs/, docstrings)
+- [ ] CI checks pass
+
+## Testing Done
+
+Describe how you tested the changes:
+
+```bash
+# commands you ran
+```
+
+## Screenshots / Output (if applicable)
+
+<!-- paste terminal output or screenshots here -->
+
+## Additional Notes
+
+Any context reviewers should know (trade-offs, known issues, follow-up work).

CODE_OF_CONDUCT.md

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+# Contributor Covenant Code of Conduct
+
+## Our Pledge
+
+We as members, contributors, and leaders pledge to make participation in our
+community a harassment-free experience for everyone, regardless of age, body
+size, visible or invisible disability, ethnicity, sex characteristics, gender
+identity and expression, level of experience, education, socio-economic status,
+nationality, personal appearance, race, religion, or sexual identity
+and orientation.
+
+We pledge to act and interact in ways that contribute to an open, welcoming,
+diverse, inclusive, and healthy community.
+
+## Our Standards
+
+Examples of behavior that contributes to a positive environment:
+
+* Demonstrating empathy and kindness toward other people
+* Being respectful of differing opinions, viewpoints, and experiences
+* Giving and gracefully accepting constructive feedback
+* Accepting responsibility and apologizing to those affected by our mistakes
+* Focusing on what is best for the overall community
+
+Examples of unacceptable behavior:
+
+* The use of sexualized language or imagery, and sexual attention or advances
+* Trolling, insulting or derogatory comments, and personal or political attacks
+* Public or private harassment
+* Publishing others' private information without explicit permission
+* Other conduct which could reasonably be considered inappropriate
+
+## Enforcement Responsibilities
+
+Community leaders are responsible for clarifying and enforcing our standards of
+acceptable behavior and will take appropriate and fair corrective action in
+response to any behavior that they deem inappropriate, threatening, offensive,
+or harmful.
+
+## Scope
+
+This Code of Conduct applies within all community spaces, and also applies when
+an individual is officially representing the community in public spaces.
+
+## Enforcement
+
+Instances of abusive, harassing, or otherwise unacceptable behavior may be
+reported to the project maintainer at [luongnv89@gmail.com](mailto:luongnv89@gmail.com).
+
+All complaints will be reviewed and investigated promptly and fairly.
+
+## Attribution
+
+This Code of Conduct is adapted from the [Contributor Covenant][homepage],
+version 2.0, available at
+https://www.contributor-covenant.org/version/2/0/code_of_conduct.html.
+
+[homepage]: https://www.contributor-covenant.org