Agent view should always be available in review viewers

[jdilla1277]

Problem

In temporary parts review viewers, the Agent view tab can be disabled with this tooltip:

Agent view is available for run viewers with embedded preview or diff images.

That is confusing and concerning during agent handoff review. It reads like the agent may not have had access to the view/state before creating the handoff, when the actual condition is narrower: the tab is currently gated on embedded preview or diff PNGs.

Current behavior

A parts review viewer with structured part review state but no embedded preview/diff images disables Agent view.

Current source gate:

• src/agentcad/commands/view.py: hasAgentImgs = PREVIEW_PNG_URL.length > 0 || DIFF_SIDE_PNG_URL.length > 0 || DIFF_OVERLAY_PNG_URL.length > 0
• setupModeButtons() disables btn-agent when !hasAgentImgs

Concrete repro from M64 friction demo:

agentcad parts view service_rover \
  --label "Payload hidden inspection" \
  --note "The payload group is hidden by default. Toggle the payload group in Part controls to compare the full assembly." \
  --hide-group payload \
  --focus-group chassis \
  --no-open

The generated viewer includes PART_REVIEW, parts, groups, colors, hidden parts, and focus state, but Agent view is disabled because no preview/diff image URLs are embedded.

Expected behavior

Agent view should always be available for review viewers, including temporary parts review viewers.

At minimum, when there are no embedded preview/diff images, Agent view should still show the agent handoff state in a human-readable way, such as:

• review label and note
• selected/focused part or group
• hidden/isolated parts and groups
• ghost-rest state
• part/group counts and colors
• a clear statement that browser-side changes are temporary and not saved

Why this matters

This tab is part of the trust surface for agent-created handoffs. If it is disabled, users may reasonably infer that the agent did not see or define the review state, even though the structured state exists in the viewer.

Acceptance criteria

• Agent view is enabled for parts review viewers even without embedded images.
• The tab presents useful structured handoff information when preview/diff images are absent.
• Existing image-based Agent view behavior still works for run viewers with preview/diff images.
• Tooltip/copy no longer implies Agent view is only for image-backed run viewers.