feat(create): add --disk-size flag

[theFong]

Summary

• add a --disk-size <GB> flag to brev create
• apply the requested size to every resolved fallback instance type
• allow an explicit disk size to override launchable storage defaults
• reject zero, negative, NaN, and infinite disk sizes
• preserve fractional sizes in create requests and dry-run output

Why

The create API already accepts diskStorage, and search/piped workflows already populate InstanceSpec.DiskGB. However, explicitly selected instance types were created with DiskGB: 0, so agents and users had no direct way to request variable storage while using --type.

Impact

Agents can now deploy a GPU with provider-supported variable storage directly:

brev create my-gpu --type g5.xlarge --disk-size 1000

The value is sent to the create API as a Gi quantity. Provider-side storage constraints continue to determine which sizes are supported.

Validation

• go build ./...
• go test ./pkg/cmd/gpucreate
• go test -race ./pkg/cmd/gpucreate
• go vet ./pkg/cmd/gpucreate
• golangci-lint run ./pkg/cmd/gpucreate

Closes #380.

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 26.07%. Comparing base (17ccd79) to head (57691c4).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/cmd/gpucreate/gpucreate.go	88.88%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #439      +/-   ##
==========================================
+ Coverage   24.67%   26.07%   +1.40%     
==========================================
  Files         134      135       +1     
  Lines       18337    18620     +283     
==========================================
+ Hits         4525     4856     +331     
+ Misses      13335    13259      -76     
- Partials      477      505      +28     

Flag	Coverage Δ
Linux	26.07% <88.88%> (+1.40%)	⬆️

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