Studio: decide whether to remove or redesign Canvas and the Execution control #2326

Part of #2302.

Summary

Two Studio controls aren't designed in the prototype the way they work today: the chat header's Canvas and the new-chat composer's Execution control. We agreed to finish the current alignment work without touching either, then decide whether to remove them or redesign them. This issue tracks that decision and the follow-up changes.

What they are today

Canvas

• A Canvas button in the chat header opens a per-chat Markdown notes panel. On phones it is under Chat options > Open local canvas.
• The panel has Edit and Preview, Add to canvas from text selected in the transcript, and Copy canvas and Clear canvas.
• The notes are stored only in the browser.
• The header button has existed since feat(studio): chat — sessions, streamed runs, controls, and the chat workspace #1756. The prototype has the same local canvas, but opens it from the chat options menu, not the header.

Execution

• The new-chat composer shows an Execution control with three kinds of choice:
  • Deployment default: the server's configured workspace;
  • No files or built-in Shell;
  • an execution template, when the server advertises a template catalog.
• When the catalog is off, it shows "Template catalog disabled; default and none remain available."
• It came with feat: add execution templates with platform-managed identity #2219 (execution templates) and is a native <select> (see Studio: use shadcn/ui primitives like the prototype and SEP (no custom components) #2201). The prototype has no equivalent.

Feedback from the design review

• Canvas:
  • It should open in relation to a source, such as a document, an image or a spreadsheet, and its view should follow that source. A free-form notepad with no context reads as a random note pad.
  • For a first version, viewing is enough; editing adds complications.
  • The suggestion is to remove the current solution.
• Execution:
  • It needs to be explained in terms a typical user understands.
  • It is an advanced setting, so it's an open question whether to show it at all.
• In general: prefer removing controls whose value is unclear and bringing them back when needed. The alternative raised was to hide controls that can't be reconciled yet behind feature flags.

Decisions needed

1. Canvas: remove the current local notepad, or keep it until a source-based canvas exists? If there will be a source-based canvas, which sources come first, view-only?
2. Execution:
   • Show it at all? If yes, where: the composer, an advanced section, or settings?
   • With which labels and copy?
   • Should it be hidden when only the default and "none" are available?
   • Settle this with the author of feat: add execution templates with platform-managed identity #2219.
3. Removal or feature flags for anything that can't be reconciled yet.

Done when

• The decisions above are recorded here.
• The resulting changes have landed. That means removing, hiding or redesigning each control, and using a primitive instead of the native select if Execution stays.
• user-docs/operating/studio.md is updated: "Choose execution for a new chat" and the canvas paragraph.

Related: #2201 (primitives; the Execution select is on hold for this) and #2262 (remaining differences from the prototype). The header Activity button came up in the same review; no decision was taken on it.