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/webui/templates/dashboard.html

@@ -35,7 +35,6 @@
       <h3>Images</h3>
       <div class="stack-inline">
         <a class="button secondary" href="/images/register">Register</a>
-        <a class="button" href="/images/build">Build</a>
       </div>
     </div>
     <table>

internal/webui/templates/images.html

@@ -3,7 +3,6 @@
   <p class="muted">Manage registered rootfs/kernel stacks and promote unmanaged experiments into daemon-owned artifacts.</p>
   <div class="stack-inline">
     <a class="button secondary" href="/images/register">Register Image</a>
-    <a class="button" href="/images/build">Build Image</a>
   </div>
 </div>
 <table>
@@ -32,48 +31,6 @@
 </table>
 {{end}}
 
-{{define "image_build_content"}}
-<p class="muted">Build a managed image from an existing registered image, then redirect into the async build progress view.</p>
-{{if .ErrorMessage}}
-<div class="inline-error">{{.ErrorMessage}}</div>
-{{end}}
-<form method="post" action="/images/build" class="form-grid">
-  {{template "csrf_field" .}}
-  <label><span>Name</span><input type="text" name="name" value="{{.ImageBuildForm.Name}}" placeholder="generated when empty"></label>
-  <label><span>From Image</span><input type="text" name="from_image" value="{{.ImageBuildForm.FromImage}}" placeholder="image id or name"></label>
-  <label><span>Size Override</span><input type="text" name="size" value="{{.ImageBuildForm.Size}}" placeholder="optional"></label>
-  <label class="picker-field">
-    <span>Kernel Path</span>
-    <div class="picker-input">
-      <input type="text" name="kernel_path" value="{{.ImageBuildForm.KernelPath}}" data-picker-input>
-      <button type="button" class="button secondary" data-picker-target="kernel_path" data-picker-kind="file">Browse</button>
-    </div>
-  </label>
-  <label class="picker-field">
-    <span>Initrd Path</span>
-    <div class="picker-input">
-      <input type="text" name="initrd_path" value="{{.ImageBuildForm.InitrdPath}}" data-picker-input>
-      <button type="button" class="button secondary" data-picker-target="initrd_path" data-picker-kind="file">Browse</button>
-    </div>
-  </label>
-  <label class="picker-field">
-    <span>Modules Directory</span>
-    <div class="picker-input">
-      <input type="text" name="modules_dir" value="{{.ImageBuildForm.ModulesDir}}" data-picker-input>
-      <button type="button" class="button secondary" data-picker-target="modules_dir" data-picker-kind="dir">Browse</button>
-    </div>
-  </label>
-  <label class="checkbox">
-    <input type="checkbox" name="docker" {{if .ImageBuildForm.Docker}}checked{{end}}>
-    <span>Install Docker</span>
-  </label>
-  <div class="form-actions">
-    <a class="button secondary" href="/images">Cancel</a>
-    <button class="button" type="submit" {{if not .MutationAllowed}}disabled{{end}}>Build Image</button>
-  </div>
-</form>
-{{end}}
-
 {{define "image_register_content"}}
 <p class="muted">Register an existing host-side image stack. Paths stay on the host; nothing is uploaded through the browser.</p>
 {{if .ErrorMessage}}

internal/webui/templates/operation.html

@@ -1,16 +1,11 @@
 {{define "operation_content"}}
 <section class="operation-card" data-operation-url="{{.OperationStatusURL}}" {{if .OperationSuccessURL}}data-operation-success="{{.OperationSuccessURL}}"{{end}}>
-  <h2>{{if eq .OperationKind "vm"}}VM readiness{{else}}Managed image build{{end}}</h2>
+  <h2>VM readiness</h2>
   {{if .VMCreateOperation}}
   <h3 id="operation-stage">{{.VMCreateOperation.Stage}}</h3>
   <p id="operation-detail">{{.VMCreateOperation.Detail}}</p>
   <p class="muted" id="operation-error">{{.VMCreateOperation.Error}}</p>
   {{end}}
-  {{if .ImageBuildOperation}}
-  <h3 id="operation-stage">{{.ImageBuildOperation.Stage}}</h3>
-  <p id="operation-detail">{{.ImageBuildOperation.Detail}}</p>
-  <p class="muted" id="operation-error">{{.ImageBuildOperation.Error}}</p>
-  {{end}}
   {{if .OperationLogPath}}
   <p class="muted">Build log: <code id="operation-log">{{.OperationLogPath}}</code></p>
   {{else}}