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/ARCHITECTURE.md

@@ -14,17 +14,16 @@ owning types:
 - `createVMMu sync.Mutex` — serialises `CreateVM` (guards name uniqueness
   + guest IP allocation window).
 - `imageOpsMu sync.Mutex` — serialises image-registry mutations
-  (`BuildImage`, `RegisterImage`, `PromoteImage`, `DeleteImage`).
+  (`PullImage`, `RegisterImage`, `PromoteImage`, `DeleteImage`).
 - `createOps opstate.Registry[*vmCreateOperationState]` — in-flight VM
   create operations; owns its own lock.
-- `imageBuildOps opstate.Registry[*imageBuildOperationState]` — in-flight
-  image build operations; owns its own lock.
 - `tapPool tapPool` — TAP interface pool; owns its own lock.
 - `sessions sessionRegistry` — active guest session controllers; owns
   its own lock.
 - `listener`, `webListener`, `webServer`, `webURL`, `vmDNS` — networking.
 - `vmCaps` — registered VM capability hooks.
-- `imageBuild`, `requestHandler`, `guestWaitForSSH`, `guestDial`,
+- `pullAndFlatten`, `finalizePulledRootfs`, `bundleFetch`,
+  `requestHandler`, `guestWaitForSSH`, `guestDial`,
   `waitForGuestSessionReady` — injectable seams used by tests.
 
 ## Subpackages