feat(theme-check-vscode): surface orphaned files on startup

[ryandiginomad]

What

Closes #970.

theme-graph already knows which files are orphaned (dead code), and there's a Liquid Theme Check: Find dead code command to list them — but it's easy to miss because you have to know to run it. This surfaces it proactively: on activation, if the theme has orphaned files, the extension shows a single dismissable notification.

• Review → opens the existing dead-code quick pick
• Don't show again → flips the setting off
• Gated by a new themeCheck.checkOrphanedFilesOnBoot setting (default true), mirroring the existing themeCheck.preloadOnBoot pattern

How

• Reuses the whole-theme dead-code detection already exposed via ThemeGraphDeadCodeRequest (ThemeGraphManager.deadCode(rootUri)) — no server changes.
• The startup check finds theme roots via workspace.findFiles('**/.theme-check.yml') (there's no active editor at boot), aggregates orphaned files across roots, and prompts once.
• Refactored makeDeadCode to extract fetchDeadCode + showDeadCodePicker so the command and the startup check share logic — makeDeadCode's behavior is unchanged and covered by a characterization test.
• Wired into both node and browser activation, fire-and-forget so it never blocks or fails activation.

Open questions (happy to adjust)

• Setting default: went with true to match the issue's "prompt on start-up" intent, but happy to make it opt-in (false) if you'd prefer less startup noise.
• Changeset bump: marked minor (new feature + setting); happy to drop to patch given the fixed version group.
• Multi-root: aggregates the count across all theme roots and "Review" opens the first root with orphans. Open to per-root prompts instead.
• Notification copy / action labels: easy to tweak to taste.

Testing

• New unit tests for the startup check (orphanedFilesOnBoot.spec.ts): disabled setting, no orphans, orphans → notification, "Don't show again" → setting update, "Review" → picker.
• Characterization tests for makeDeadCode to guard the refactor.
• Full theme-check-vscode vitest suite green (23 tests), tsc --noEmit clean, prettier clean.

🤖 AI assistance

Drafted with Claude Code. I reviewed every change, ran the tests, and verified the approach against existing extension patterns. The orphaned-files detection this builds on is existing Shopify code; this PR adds the startup surfacing plus a shared refactor.