docs: Add PR template changelog workflow

[yunseo-kim]

Summary

• Add a dedicated Changelog section to the organization PR template.
• Guide contributors to record whether each PR needs user-facing release notes.
• Document Keep a Changelog categories, the Unreleased-first development flow, release PR promotion steps, comparison-link updates, and Human Era five-digit release date examples.
• Reflect the workflow in CONTRIBUTING.md so organization projects know when and how to update CHANGELOG.md.
• Update the repository inventory to describe the PR template's changelog decision field.

Related Issues

• Closes #
• Related #

Change Type

• Bug fix
• Feature
• Refactor
• Documentation
• Test/CI
• Breaking change
• Other:

Changelog

• Category: Changed
• User-facing note: PR templates and contribution guidance now ask contributors to make explicit changelog decisions and update CHANGELOG.md [Unreleased] for meaningful user-facing changes.

Changelog update:

• CHANGELOG.md [Unreleased] updated
• Not needed because this repository does not currently maintain a product CHANGELOG.md; this PR defines the organization-wide workflow.

Checklist

General

• PR title follows Conventional Commits format: docs: Add PR template changelog workflow
• This PR does not expose backend/internal implementation details in a public repo.
• No secrets, tokens, keys, or private endpoints are included.
• Changes stay within this repository's intended scope.

CI/Workflow Changes (if applicable)

Not applicable.

• All uses: references are pinned to full 40-character commit SHAs (with # vX.Y.Z comment)
• step-security/harden-runner is included as the first step in every job
• Job-level permissions are used instead of top-level permissions

Protocol / Compatibility Impact

• No protocol/spec impact
• Protocol/spec updated
• Conformance tests updated
• Breaking change is versioned and migration notes are included

If impacted, describe compatibility impact: N/A

Testing

• Unit tests added/updated
• Integration or conformance tests added/updated
• Tests pass
• Lint and format pass
• Type check passes
• Manual verification performed

Describe test evidence:

• bun run lint:md
• bunx prettier --check CONTRIBUTING.md README.md .github/PULL_REQUEST_TEMPLATE.md
• Pre-commit Markdown lint/format hook passed during commits.
• Manually re-read the edited template and contributing-guide sections and confirmed the Human Era date example is present.

Documentation

• README updated
• Spec/docs updated
• Changelog decision completed above

Rollout / Risk

• Risk level: Low
• Rollback plan: Revert the documentation commits if the workflow is too strict or needs rewording.

Reviewer Checklist

• Scope is clear and minimal
• Security and boundary checks passed
• Tests and docs are sufficient
• Compatibility impact is correctly handled