docs: update /debug → /investigate references in docs

CLAUDE.md, README.md, and docs/skills.md still referenced the old
/debug skill name after the rename.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/skills.md

@@ -10,7 +10,7 @@ Detailed guides for every gstack skill — philosophy, workflow, and examples.
 | [`/plan-design-review`](#plan-design-review) | **Senior Designer** | Interactive plan-mode design review. Rates each dimension 0-10, explains what a 10 looks like, fixes the plan. Works in plan mode. |
 | [`/design-consultation`](#design-consultation) | **Design Partner** | Build a complete design system from scratch. Knows the landscape, proposes creative risks, generates realistic product mockups. Design at the heart of all other phases. |
 | [`/review`](#review) | **Staff Engineer** | Find the bugs that pass CI but blow up in production. Auto-fixes the obvious ones. Flags completeness gaps. |
-| [`/debug`](#debug) | **Debugger** | Systematic root-cause debugging. Iron Law: no fixes without investigation. Traces data flow, tests hypotheses, stops after 3 failed fixes. |
+| [`/investigate`](#investigate) | **Debugger** | Systematic root-cause debugging. Iron Law: no fixes without investigation. Traces data flow, tests hypotheses, stops after 3 failed fixes. |
 | [`/design-review`](#design-review) | **Designer Who Codes** | Live-site visual audit + fix loop. 80-item audit, then fixes what it finds. Atomic commits, before/after screenshots. |
 | [`/qa`](#qa) | **QA Lead** | Test your app, find bugs, fix them with atomic commits, re-verify. Auto-generates regression tests for every fix. |
 | [`/qa-only`](#qa) | **QA Reporter** | Same methodology as /qa but report only. Use when you want a pure bug report without code changes. |
@@ -440,9 +440,9 @@ I want the model imagining the production incident before it happens.
 
 ---
 
-## `/debug`
+## `/investigate`
 
-When something is broken and you don't know why, `/debug` is your systematic debugger. It follows the Iron Law: **no fixes without root cause investigation first.**
+When something is broken and you don't know why, `/investigate` is your systematic debugger. It follows the Iron Law: **no fixes without root cause investigation first.**
 
 Instead of guessing and patching, it traces data flow, matches against known bug patterns, and tests hypotheses one at a time. If three fix attempts fail, it stops and questions the architecture instead of thrashing. This prevents the "let me try one more thing" spiral that wastes hours.