feat(coc): Agents canvas follow-ups — hide toggle without sub-agents + fix overlay scroll

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Two follow-up improvements to the Agents canvas (the spatial sub-agent run tree shipped in #326).

1. Hide the Thread/Agents toggle until the chat has sub-agents

feat(coc): hide Thread/Agents toggle until the chat has sub-agents

The Agents canvas is only meaningful once a chat has actually spawned sub-agents, so showing the Thread | Agents toggle on a plain chat was noise. ChatDetail now derives hasSubAgents from the agent-run tree and:

• hides the toggle when there are no sub-agents (on top of the existing loading/pending/floating guards), and
• renders from a new effectiveView (= view only when sub-agents exist, otherwise thread). This means a stale ?view=agents deep-link on a chat without sub-agents can't strand the user on an empty canvas with no toggle to escape — it falls back to the thread and "waits", revealing the canvas the moment the first sub-agent appears.

The thread-only flow cards (Ralph start, Implement-plan) also key off effectiveView, so they still show on the thread fallback.

2. Don't zoom the canvas when scrolling inside an overlay

fix(coc): don't zoom the canvas when scrolling inside an overlay

The wheel-to-zoom handler in the shared useZoomPan hook fired for any wheel event bubbling up to the canvas container — and the AgentInspector panel renders inside that container. So scrolling the inspector's content zoomed the canvas instead of scrolling the panel.

The wheel handler now bails when the event originates inside a [data-no-drag] region — the same opt-out the pan-drag handler already honored. The inspector already carries data-no-drag on its overflow-y: auto element, so its content now scrolls natively while the canvas surface still zooms as before. As a bonus, the toolbar/legend (and the DAG charts' zoom controls, which also use data-no-drag) no longer trigger an accidental zoom when scrolled over.

Testing

• New: ChatDetail-agents-toggle.test.ts — static-analysis test for the toggle/effectiveView wiring (matching the repo's existing ChatDetail source-test convention; the component is too hook/context/SSE-heavy to render).
• New: real-DOM wheel cases in useZoomPan.test.ts — the existing tests poke a mock ref and never attach the native wheel listener; the new ones cover zoom-over-surface (with preventDefault) and no-zoom-inside-[data-no-drag]-overlay.
• Reran the agent-canvas suite and both useZoomPan consumers' suites (WorkflowDAGChart ×2, all DAG tests) — all green.
• npm run build:client clean; eslint reports 0 errors on changed files.

Reviewer notes

• Touches the shared useZoomPan hook. The change is additive (only affects elements that opt in with [data-no-drag]); behavior for the WorkflowDAG consumers is unchanged except that scrolling directly over their already-data-no-drag zoom controls no longer zooms — a strict improvement.
• coc-knowledge dashboard-spa.md reference updated for both behaviors.

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