chore: improve spec template based on S04 authoring feedback
## Context
Feedback from @suleimiahmed while authoring the S04 spec (MR !46), posted on MR !35.
## Improvements
### 1. Add a "Scope type" field to the spec template
`feature | infrastructure | tooling`
Infrastructure specs naturally have empty API Contracts and Data Model sections. A scope type would set expectations upfront so authors do not need to justify empty sections.
### 2. Add blocking/non-blocking indicators to Open Questions
Open questions currently sit in a flat list with no severity distinction. A blocking question prevents implementation from starting; a non-blocking question can be resolved during implementation. Something like `[blocking]` / `[non-blocking]` tags.
### 3. Add a "what needs a spec" section to `docs/dev/README.md`
A decision tree to help contributors decide when a spec is required:
- Feature: spec required
- Infrastructure/tooling: spec recommended, plan file may suffice
- Bugfix/chore: neither
## Files
- `docs/specs/TEMPLATE.md`
- `docs/dev/README.md`
issue