Remove image build --from-image; doctor treats catalog images as OK

The `image build` flow spun up a transient Firecracker VM, SSHed in,
and ran a large bash provisioning script to derive a new managed
image from an existing one. It overlapped heavily with the golden-
image Dockerfile flow (same mise/docker/tmux/opencode install logic
duplicated in Go as `imagemgr.BuildProvisionScript`) and had far more
machinery: async op state, RPC begin/status/cancel, webui form +
operation page, preflight checks, API types, tests. For custom
images, writing a Dockerfile is simpler and more reproducible.

Removed end-to-end:
- CLI `image build` subcommand + `absolutizeImageBuildPaths`.
- Daemon: BuildImage method, imagebuild.go (transient-VM orchestration),
  image_build_ops.go (async begin/status/cancel), imagemgr/build.go
  (the 247-line provisioning script generator and all its append*
  helpers), validateImageBuildPrereqs + addImageBuildPrereqs.
- RPC dispatches for image.build / .begin / .status / .cancel.
- opstate registry `imageBuildOps`, daemon seam `imageBuild`,
  background pruner call.
- API types: ImageBuildParams, ImageBuildOperation, ImageBuildBeginResult,
  ImageBuildStatusParams, ImageBuildStatusResult; model type
  ImageBuildRequest.
- Web UI: Backend interface methods, handlers, form, routes, template
  branches (images.html build form, operation.html build branch,
  dashboard.html Build button).
- Tests that directly exercised BuildImage.

Doctor polish (task C):
- Drop the "image build" preflight section entirely (its raison d'être
  is gone).
- Default-image check now accepts "not local but in imagecat" as OK:
  vm create auto-pulls on first use. Only flag when the image is
  neither locally registered nor in the catalog.

Net: 24 files touched, 1,373 lines deleted, 25 added.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/daemon/doc.go

@@ -12,7 +12,7 @@
 // Subpackages:
 //
 //	internal/daemon/opstate    Generic Registry[T AsyncOp] for async
-//	                           operations (VM create, image build).
+//	                           operations (VM create).
 //	internal/daemon/dmsnap     Device-mapper COW snapshot lifecycle.
 //	internal/daemon/fcproc     Firecracker process helpers: bridge/tap,
 //	                           binary resolution, PID lookup, wait/kill.
@@ -46,8 +46,7 @@
 // Image management (in this package):
 //
 //	images.go             register, promote, delete, find, list
-//	imagebuild.go         orchestrates the transient firecracker build VM
-//	image_build_ops.go    async begin/status/cancel (uses opstate.Registry)
+//	images_pull.go        image pull: catalog (bundle) + OCI paths
 //	image_seed.go         managed work-seed SSH fingerprint refresh
 //
 // Guest interaction (in this package):