[FEATURE] Improve create reliability: pre-flight validation + better error messages for instance type failures

[dims]

Summary

brev create failures are opaque. The same error message — instance type X not found — covers four completely different failure modes, giving users no direction for fixing the problem.

Additionally, the CLI sends the create request without first checking whether the instance type is valid, so avoidable failures require a full round-trip (plus a ~10 second spinner) before surfacing.

The problem

All four of these produce identical output:

Actual cause	Error shown
Type string is a genuine typo	not found
Type exists but CLI is missing workspaceGroupId (see #15)	not found
Type exists but no live capacity right now	not found
Type exists but org plan doesn't include it	not found

A user who sees not found for hyperstack_H100 — a valid, available type — has no path forward without reading Go source code.

Requested: client-side pre-flight

Before calling CreateWorkspace, resolve the instance type through the catalog API and fail fast with an actionable error:

$ brev create my-gpu --type hyperstack_H100

Error: instance type 'hyperstack_H100' could not be provisioned.
Your CLI (v0.6.316) does not pass the required workspaceGroupId for this provider.

  brew upgrade brev

If the issue persists, run 'brev search gpu' to see alternatives.

Pre-flight also naturally populates workspaceGroupId — fixing #15 as a side effect.

Requested: server-side gRPC status codes

Return distinct codes per failure mode so client error messages can be targeted:

Cause	Code	Message
Type string unknown	NotFound	"unknown instance type 'X'. Run 'brev search gpu'"
workspaceGroupId missing/wrong	InvalidArgument	"type 'X' requires workspaceGroupId. Update your CLI."
No live capacity	ResourceExhausted	"no capacity for 'X' right now. Alternatives: ..."
Plan restriction	PermissionDenied	"type 'X' not included in your plan"

Why this matters

Error message quality is the cheapest possible improvement to developer experience — no architecture changes, no new features. The user who sees "update your CLI" spends 30 seconds. The user who sees "not found" spends hours.

CLI version: v0.6.316