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 repo gains the .github/pull_request_template.md that AGENTS.md already referenced, with a Before / After section that asks for proof — CLI/API output, logs, metrics, or screenshots for UI. The inline PR-body template in AGENTS.md is updated to match (and a couple of typos fixed).

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: AGENTS.md referenced a template file that didn't exist; inline body used ## What / ## How.
• After: ## What changed (functional summary + impact) + ## Before / After (proof: output/logs/metrics, screenshots for UI when possible); template file now exists and matches AGENTS.md.

Risk

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

Checklist

• Unit tests are passed — N/A (docs/templates only)
• Smoke tests are passed — N/A
• Documentation is updated
• Specs are up to date and not in conflict

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