[ghcp-handoff] T8: Expand slash-command triggers for natural phrasings

[bminier]

Problem

Triggers list: "hand off to copilot", "delegate to ghcp", "ghcp-handoff", "scaffold handoff", "mechanical refactor handoff", "copilot prompt". Common natural phrasings that miss: "use copilot for this", "send this to copilot", "have copilot do X", "copilot should handle this".

Approach

Add a dozen more synonyms to the `triggers:` and `voice-triggers:` frontmatter arrays. Cheap change, high recognition uplift.

Review reference

Adversarial review T8

[bminier]

Fixed on branch ghcp-t8-expand-triggers (stacked on ghcp-t1-heading-aliases). Added 16 new natural-phrasing triggers to the skill frontmatter. Regression test pins representative cases so future trims don't undo the work.