chore: add AI tool policy to CONTRIBUTING.md & update PR template

[clavin]

Description of Change

Adds links to the AI tool policy in the contributors guide and the PR template. I also took the opportunity to polish some words & squish a couple checklist items under one umbrella.

Additional Context

• The policy was accepted in RFC 26
• Depends on doc: AI Tool Policy governance#613 for links to its main branch.

Checklist

• PR release notes describe the change in a way relevant to app developers, and are capitalized, punctuated, and past tense.

Release Notes

Notes: none

[clavin]

i recognize that some folks might find value in having these in the template. i decided to consolidate them. the rationale is redundancy--we have concrete checks for some, and it feels like these are all different forms of us communicating the same one thing: 'are you contributing respectfully?'

(2¢: they became noisy over time with additions and makes this section's friction too high)

[nikwen]

I'd vote against separating those.

I have built and tested this PR was added because we had the other two checks and people were still submitting stuff that they hadn't built.

Also, npm test is different from manual testing. And being able to explain the changes is different from including a description.

I think verbosity is good here. It communicates our expectations clearly, and the more opportunities we give people to self-report, the better.

It takes 5 seconds to fill out the template, but it might save a reviewer 15 minutes of reviewing a crappy PR that doesn't even build.

In my opinion, the math is in favor of verbosity here.

[nikwen]

I'd vote against separating those.

I have built and tested this PR was added because we had the other two checks and people were still submitting stuff that they hadn't built.

Also, npm test is different from manual testing. And being able to explain the changes is different from including a description.

I think verbosity is good here. It communicates our expectations clearly, and the more opportunities we give people to self-report, the better.

It takes 5 seconds to fill out the template, but it might save a reviewer 15 minutes of reviewing a crappy PR that doesn't even build.

In my opinion, the math is in favor of verbosity here.