fix: preserve path separators in create command argument

[srtaalej]

Summary

• Fixes the slack create path/to/my-app regression where path separators were being replaced with dashes, creating a flat path-to-my-app directory instead of preserving the path/to/my-app structure
• Adds parseAppPath helper that splits user input into path prefix + basename, only kebab-casing the basename
• Display name for manifests is derived from the raw basename (not the full path)

Behavior

$ slack create my-app              # dir: ./my-app/        manifest: "my-app"
$ slack create "My App"            # dir: ./my-app/        manifest: "My App"
$ slack create path/to/my-app      # dir: ./path/to/my-app/ manifest: "my-app"
$ slack create path/to/My App      # dir: ./path/to/my-app/ manifest: "My App"

Stacks on #521 which handles the display name vs dir name separation.

Test plan

• TestParseAppPath — 12 cases covering simple names, paths, absolute paths, dot-prefixed, trailing slashes, and error cases
• Existing TestGetProjectDirectoryName still passes (single-component names)
• Full test suite (go test ./...) passes
• Manual: slack create path/to/my-app with existing path/to/ directory → verify nested structure created and manifest has my-app
• Manual: slack create "My App" → verify dir is my-app, manifest display is My App

[codecov]

Codecov Report

❌ Patch coverage is 68.42105% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.31%. Comparing base (8b4871f) to head (fca83ad).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/pkg/create/create.go	68.42%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #526      +/-   ##
==========================================
+ Coverage   71.28%   71.31%   +0.02%     
==========================================
  Files         222      222              
  Lines       18698    18711      +13     
==========================================
+ Hits        13329    13343      +14     
  Misses       4188     4188              
+ Partials     1181     1180       -1     

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