Keep Foundry scenario skill descriptions within 1,024 characters

[JeffreyCA]

The description frontmatter in all three foundry-extension-scenario-* skills exceeds GitHub Copilot's 1,024-character limit:

• .github/skills/foundry-extension-scenario-authoring/SKILL.md: 1,334 characters
• .github/skills/foundry-extension-scenario-pr-regression/SKILL.md: 1,300 characters
• .github/skills/foundry-extension-scenario-suite-run/SKILL.md: 1,342 characters

These descriptions were added with #9342. They are routing metadata that Copilot loads before deciding whether to invoke a skill. Oversized descriptions can be rejected or omitted, which makes the skills unavailable for model-directed invocation.

An audit of the 10 current .github/skills/*/SKILL.md files found no other descriptions over the limit.

Proposed change

• Shorten the three descriptions to at most 1,024 characters after YAML parsing.
• Preserve the distinct routing cases:
  • author or edit a scenario without running it
  • run scenarios selected from a PR or diff
  • run the full suite or a tag or tier subset
• Keep the key anti-triggers, especially Azure cost boundaries and the distinction between authoring, PR regression, and suite execution.
• Move procedural detail from description into the skill body or references where needed.
• Optionally, add a repository check that parses every .github/skills/*/SKILL.md frontmatter block and fails when description exceeds 1,024 characters. The failure should report the file and parsed character count.

Acceptance criteria

• Each foundry-extension-scenario-* description is at most 1,024 characters after YAML parsing.
• The descriptions still route the three distinct workflows correctly and do not collapse them into overlapping triggers.