[Ticket]: Add a SOLID/DRY reviewer checklist to the PR template

### Type
Technical debt

### Summary
Extend the PR template with the specific review questions the linter cannot answer.

### Intent
So review attention lands on failure modes and ownership shape rather than on formatting the linter already enforces.

### Source of truth (links)
- `AGENTS.md`, 'Spend review attention here instead'
- https://adorsys-gis.github.io/ai-governance/04-pull-request-template

### Current Behavior
The template carries governance sections but no design-quality prompts.

### Expected Behavior
Reviewers are asked the five questions that have actually caught bugs here.

### Acceptance Criteria
- [ ] Checklist covers: fail-closed behaviour, ownership/cloning, error-type levels, whether tests would fail if logic were wrong, and locks held across `.await`
- [ ] Existing governance sections are preserved unchanged
- [ ] Each item asks for a MECHANISM, not a verdict

### Out of Scope
Automated enforcement of any of it.

### Technical Context
The first question is the highest-yield one in this repo: does the unavailable branch become the permissive branch?

### Risks
A long checklist gets skimmed. Keep it to five items.

### Test Plan
Apply to one real PR and confirm the prompts produce concrete comments.

### Verification evidence
- [ ] Test added and proven to fail before the change
- [ ] `just all-checks` green
- [ ] Every new or changed `src` file is <= 200 LoC

### Human accountable owner
@stephane-segning

### AI Usage Declaration
Ticket drafted with AI assistance from a verified repo audit.
Structure and estimates drafted with AI from a verified repository audit (file
line counts, config keys, and dependency state were read from the actual tree,
not assumed). A human owns intent, scope, and the release commitment.

### Human verification completed
- [ ] I can explain this work without referring to the AI-generated text
- [ ] Acceptance criteria are testable
- [ ] Source of truth is a real link, not boilerplate

Governance: https://adorsys-gis.github.io/ai-governance/