docs: add Why/What/How structure to PR template, CLAUDE.md, and PR skills (#12525)

Requested by @majdyz

### Why / What / How

**Why:** PR descriptions currently explain the *what* and *how* but not
the *why*. Without motivation context, reviewers can't judge whether an
approach fits the problem. Nick flagged this in standup: "The PR
descriptions you use are explaining the what not the why."

**What:** Adds a consistent Why / What / How structure to PR
descriptions across the entire workflow — template, CLAUDE.md guidance,
and all PR-related skills (`/pr-review`, `/pr-test`, `/pr-address`).

**How:**
- **`.github/PULL_REQUEST_TEMPLATE.md`**: Replaced the old vague
`Changes` heading with a single `Why / What / How` section with guiding
comments
- **`autogpt_platform/CLAUDE.md`**: Added bullet under "Creating Pull
Requests" requiring the Why/What/How structure
- **`.claude/skills/pr-review/SKILL.md`**: Added "Read the PR
description" step before reading the diff, and "Description quality" to
the review checklist
- **`.claude/skills/pr-test/SKILL.md`**: Updated Step 1 to read the PR
description and understand Why/What/How before testing
- **`.claude/skills/pr-address/SKILL.md`**: Added "Read the PR
description" step before fetching comments

## Test plan
- [x] All five files reviewed for correct formatting and consistency

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Co-authored-by: Zamil Majdy (@majdyz) <zamil.majdy@agpt.co>