chore: standardize PR descriptions on functional change + before/after

[chaliy]

What changed

PR descriptions now center on functional change and impact instead of a code-location walkthrough, and the PR template gains a Before / After section that asks for proof — CLI/API output, logs, metrics, or screenshots for UI. The same standard is applied to both .github/pull_request_template.md and the inline PR-body template in AGENTS.md.

Why

The diff already shows where and how code changed. A description should add what the diff can't: the behavior that changed, who it affects, and evidence it works — shifting effort from restating code to demonstrating impact.

Before / After

Docs/templates only — no runtime behavior changes.

• Before: ## What ("Clear description of the change") + ## How ("High-level approach").
• After: ## What changed (functional summary + impact) + ## Before / After (proof: output/logs/metrics, screenshots for UI when possible).

Risk

• Low
• Documentation and PR-template only; no source, config, or CI logic touched.

Checklist

• Tests added or updated — N/A (docs/templates only)
• Backward compatibility considered — no code paths affected

---

Generated by Claude Code