docs: add community health files

[kitsuyui]

Summary

Add missing community health files to make contributing and security reporting explicit.

Changes

• CONTRIBUTING.md: documents how to set up the project (bun install), the required checks before a PR (lint, test, build), and the code style tool (Biome)
• SECURITY.md: security policy with a private vulnerability disclosure path via GitHub Security Advisories; scope covers token usage, unintended write access, and dependency vulnerabilities
• .github/ISSUE_TEMPLATE/bug_report.md: structured bug report template for action issues
• .github/ISSUE_TEMPLATE/feature_request.md: feature request template
• .github/PULL_REQUEST_TEMPLATE.md: PR checklist for lint, test, build, and test coverage

Motivation

The repository lacked any community health files, leaving contributors without guidance on the development process and security disclosure path. This adds the minimum set to address those gaps.

Verification

All added files are Markdown only. No source code, workflow logic, or action.yml was changed.

[github-actions]

🎉 Happy commit!

• dogfooding: PR docs: add community health files #846 always celebrates.
• dogfooding: d1938b89908c915658e133c3c3789b692ef7f771 always celebrates.

[github-actions]

Code Metrics Report

	main (8bad195)	#846 (7d7d921)	+/-
Coverage	99.2%	99.2%	0.0%
Code to Test Ratio	1:1.3	1:1.3	0.0
Test Execution Time	2s	2s	0s

Details

  |                     | main (8bad195) | #846 (7d7d921) | +/-  |
  |---------------------|----------------|----------------|------|
  | Coverage            |          99.2% |          99.2% | 0.0% |
  |   Files             |              5 |              5 |    0 |
  |   Lines             |            134 |            134 |    0 |
  |   Covered           |            133 |            133 |    0 |
  | Code to Test Ratio  |          1:1.3 |          1:1.3 |  0.0 |
  |   Code              |            723 |            723 |    0 |
  |   Test              |            955 |            955 |    0 |
  | Test Execution Time |             2s |             2s |   0s |

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Reported by octocov