[Change] Copilot Test Generation for CLI Scenarios

[djm81]

Why

SpecFact already has a generate test-prompt workflow for creating tests using IDE copilots. Extending this to generate CLI behavior scenarios means every new feature ships with end-user validation from day one — without developers manually writing YAML. This makes the "patterns + anti-patterns as first-class artifacts" vision operationally sustainable.

What Changes

• NEW: Prompt template in resources/prompts/ generating scenario YAML, anti-pattern catalog, and Markdown acceptance tests
• EXTEND: specfact generate test-prompt workflow to detect CLI commands and offer scenario template
• EXTEND: OpenSpec convention requiring scenario files for CLI command changes
• EXTEND: Documentation with copilot-driven scenario authoring workflow

Acceptance Criteria

• Prompt template generates valid YAML conforming to cli-val-01 schema
• Template tested against 3 example commands
• generate test-prompt detects CLI commands and offers scenario template
• Convention documented in contributor guide

Blocked by: #279 (cli-val-01). Soft dependency: #283 (cli-val-05) for enforcement.

OpenSpec Change Proposal: cli-val-06-copilot-test-generation