chore: add contribution and repository governance guidelines

[2dubu]

Summary

• Add a default pull request template centered on background, root-cause analysis, verification, compatibility, and performance impact.
• Add contribution guidelines with CI-equivalent commands and expectations for deterministic, cause-based regression tests.
• Add a repository-wide CODEOWNERS entry for @2dubu.
• Link the contribution guide from the README.

Background

PaletteKit is receiving external contributions but did not yet document its review and testing expectations. In particular, bug-fix pull requests should explain the underlying failure mechanism and verify the relevant invariant rather than relying only on a reported attachment or a no-crash assertion.

Repository governance changes should also remain separate from package implementation work so they can be reviewed and tracked independently.

Solution

Introduce a small set of repository-level guidance:

• one concise PR template rather than separate templates for each change type;
• a CONTRIBUTING.md covering development requirements, test commands, root-cause regression testing, performance-sensitive changes, and API compatibility;
• a simple CODEOWNERS rule; and
• a README link that makes the guide discoverable.

Related Issue

N/A

Verification

• Reviewed the complete staged diff and confirmed that it contains only the four intended governance/documentation files.
• Ran git diff --check with no errors.
• Swift tests were not run locally because this PR changes no package source, tests, manifest, or runtime behavior; the repository CI remains the merge gate.

Compatibility and Impact

• Public API: None
• Behavioral impact: None
• Performance impact: None
• Affected platforms: None; repository process only

Additional Context

This change does not require a package version bump, changelog entry, tag, or release. The PR template and CODEOWNERS rule take effect after this PR is merged into the default branch. The proposed main ruleset and merge settings are GitHub repository settings and will be configured separately.

Checklist

• This pull request is focused and contains no unrelated changes.
• Tests were added or updated where behavior changed, or I explained why they are not needed.
• Regression tests cover the intended behavior or root cause rather than only a reported sample.
• Public API changes are documented and reflected in the changelog, or this pull request has none.
• I considered source compatibility, behavioral changes, and performance impact.