feat: add PR requirements spec and PR template

[diberry]

Part 1 of 2: Repo Health -- PR Requirements

This PR establishes the rules. Automated enforcement comes next.

What

Adds the canonical PR requirements spec and PR template for the Squad repo.

Why

Establishes PR quality standards as versioned files. Phase 2 delivers them as versioned files.
Addresses Challenger FATAL finding: spec must be versioned, not mutable issue.

How

• .github/PR_REQUIREMENTS.md: versioned spec (6 categories, waiver process, exemptions)
• .github/PULL_REQUEST_TEMPLATE.md: author-facing checklist

Related Issues

• PR requirements PRD (this PR delivers Phase 2)
• PR completeness gates (Part 2: automated enforcement)
• Reviewer checklist (Phase 4)

Testing

• npm run build passes
• npm test passes (179/183 suites green; 4 failures are vitest worker timeouts unrelated to markdown-only changes)
• No code changes -- markdown files only

Docs

N/A -- these ARE the docs

Breaking Changes

None

Waivers

None

[Copilot]

Pull request overview

Adds repository-wide pull request governance documentation under .github/, aiming to standardize PR quality requirements and provide a contributor-facing checklist.

Changes:

• Introduces a versioned PR requirements specification (.github/PR_REQUIREMENTS.md).
• Adds a PR author checklist template (.github/PULL_REQUEST_TEMPLATE.md).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 5 comments.

File	Description
.github/PR_REQUIREMENTS.md	Defines PR requirement categories, waiver/exemption rules, and “user-facing change” criteria.
.github/PULL_REQUEST_TEMPLATE.md	Provides a structured PR description template and checklists aligned to the spec.

[diberry]

@bradygaster -- I'm working on getting Squad to interact with Copilot code review correctly. Most of Copilot's suggestions and comments resulted in changes to this PR. CI is green and this is ready to merge when you are.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

[Copilot]

The Docs checklist says a CHANGELOG entry is needed when packages/squad-sdk/src/ or packages/squad-cli/src/ changed, but PR_REQUIREMENTS.md scopes CHANGELOG requirements to user-facing changes (public surface changes), not all edits under those paths. Align the template language with the spec (e.g., reference the “user-facing change” definition) to avoid conflicting guidance.

Suggested change

	- [ ] CHANGELOG.md entry (if packages/squad-sdk/src/ or packages/squad-cli/src/ changed)
	- [ ] CHANGELOG.md entry (for user-facing changes in packages/squad-sdk/src/ or packages/squad-cli/src/; see PR_REQUIREMENTS.md “user-facing change” definition)