De-squad the PR template (.github/pull_request_template.md)

[primetimetank21]

.github/pull_request_template.md still encodes the squad workflow and references removed squad infrastructure:

• Line ~23: checklist item referencing .squad/agents/{name}/history.md
• Line ~29: "Branch forked from develop (not from another squad/* branch)"
• Line ~30: "No rogue files outside the canonical .squad/ paths ..."

Rewrite into a generic contributor PR template with no squad-specific checklist items.

Acceptance criteria:

• No .squad/ or squad/* references remain in .github/pull_request_template.md

Follow-up to #475 (embedded-squad removal).