Gate Wizard canvas generation behind an opt-in open input

[nicolehaugen]

Summary

• Hide Generate canvas by default and expose it only when the Wizard is opened with generateCanvas: true.
• Load the Designer handoff handler only when the launch endpoint is invoked, so the Wizard can start without Designer files or an enabled Designer provider.
• Remove unreleased canvas-generation guidance from the Wizard-related READMEs.

Validation

• Existing Wizard dialog, snapshot, and Designer launch tests pass (28 tests).
• Session-local Wizard preview opened both without Designer's sibling helper and with the opt-in enabled.

[Copilot]

Copilot review overview

🟡 Changes recommended

The central default-off behavior lacks a regression test.

Review effort: Balanced
Findings: 1

Open (1)

• Add coverage for default-off pipeline generation · New

What changed in this PR

Adds an opt-in gate for experimental Wizard canvas generation while decoupling Wizard startup from Designer files.

Changes:

• Propagates generateCanvas from canvas input to the UI.
• Lazily imports the Designer launch handler.
• Removes unreleased generation guidance.

File	Description
README.md	Removes future handoff messaging.
ui/​phase-runtime.js	Conditionally renders Generate canvas.
test/​canvas-designer-dialog.test.mjs	Enables the flag in the existing test.
server.mjs	Lazily loads the Designer launch handler.
speckit-wizard-canvas/​README.md	Removes experimental workflow documentation.
extension.mjs	Declares and captures the opt-in input.
e2e/​server.mjs	Enables generation in the E2E fixture.
canvas-runtime/​snapshot.mjs	Exposes the feature flag to clients.
speckit-canvas-designer/​README.md	Simplifies Designer shell documentation.

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