Remove `build:prompts` step from CI pipelines

[scriptease]

After #798 is merged, the build:prompts step (npm run build:prompts) is a no-op — prompts are now loaded from disk at runtime instead of being compiled into src/generated/prompts.ts at build time.

A no-op shim was added to package.json (7189b06c) so existing CI pipelines don't break, but the step should be removed from CI configs entirely since it does nothing.

Relates to #197.

What to do

1. Search CI workflow files (GitHub Actions, etc.) for references to build:prompts or generate-prompts
2. Remove those steps
3. Remove the no-op "build:prompts" script from package.json

Context

• scripts/generate-prompts.mjs was deleted
• src/generated/prompts.ts no longer exists
• src/generated/ was removed from .gitignore
• The build, dev:main, dev:main:prod-data, and validate:push scripts already had their build:prompts references removed in Core prompts #798
• The no-op shim only exists for external/cached CI pipelines that haven't been updated yet

[pedramamini]

Thanks for filing this cleanup task, @scriptease!

This is a valid follow-up, but it's blocked on #798 (which migrates prompts to disk-based loading). That PR is still open — until it merges, build:prompts is still a real build step that generates src/generated/prompts.ts.

Once #798 lands, the cleanup here is straightforward:

1. Remove npm run build:prompts from .github/workflows/ci.yml (lines 19 and 33)
2. Remove the "build:prompts" no-op shim from package.json
3. Clean up any remaining references in BUILDING_WINDOWS.md, scripts/start-dev.ps1, and src/prompts/index.ts

I'll pick this up as soon as #798 merges. Leaving this open and tracking it as a follow-up.