docs: add security policy, code of conduct, and update contributing guidelines (#67)

[PandaHUN777]

Description

Addresses community health checklist items in #67 by establishing standard security disclosure guidelines, adopting the Contributor Covenant Code of Conduct, and connecting them with the contributing guide and changelog.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Performance improvement
• Code refactoring
• Test improvement

Related Issues

Fixes #67

Changes Made

• Added SECURITY.md defining supported minor release lines and confidential reporting via GitHub Private Vulnerability Reporting.
• Added CODE_OF_CONDUCT.md adopting Contributor Covenant v2.1.
• Updated CONTRIBUTING.md to reference CODE_OF_CONDUCT.md and direct security reports to SECURITY.md / GitHub Private Vulnerability Reporting.
• Added entry to CHANGELOG.md under ## [Unreleased].

Testing

Test Coverage

• All existing tests pass (go test ./...)

Documentation

• CHANGELOG updated

Code Quality

• Code follows the project's style guidelines

Breaking Changes

Does this PR introduce breaking changes?

• Yes
• No

Performance Impact

• No performance impact

Checklist

• I have read the CONTRIBUTING guidelines
• My code follows the code style of this project
• All tests pass locally
• I have updated the documentation accordingly
• I have added an entry to CHANGELOG.md

[coderabbitai]

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This repository does not receive automatic reviews because it has fewer than 10 stars.

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Configuration used: defaults

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Plan: Advanced

Run ID: 4ad7073e-6e6d-44f7-b08b-718b1c7a7a74

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