Port Easel background Tweaks workflow

[jamesrochabrun]

What changed

• run Ideas and free-form tweak generation through a dedicated ephemeral Claude or Codex agent
• edit an isolated single-file workspace and apply results only when the source still matches its starting snapshot
• preserve existing controls for additive Ideas requests and report progress, no-change results, and concurrent-edit conflicts
• keep live tweak values in memory until reload and add explicit Reset and Save as defaults actions
• write changed defaults atomically while rejecting source or schema drift
• show the Tweaks loading state and preserve compatible schemas across page reloads
• pin Canvas and both lockfiles to exact release 1.3.2

Why

Sending tweak generation through the visible session blocks the main conversation, while running another agent directly in the project risks overwriting concurrent edits. Persisting every live control interaction also couples experimentation to source changes.

This ports the isolated background-agent workflow from Easel #95 and the explicit defaults workflow from Easel #96 to AgentHub.

Impact

Users can keep chatting while controls are generated, safely retry when the target changes concurrently, experiment without changing source defaults, reset live values, and explicitly promote the current values to defaults.

Validation

• xcodebuild test -scheme AgentHubCore-Tests -destination platform=macOS -test-timeouts-enabled YES -skipPackagePluginValidation with focused -only-testing filters
• 16 tests passed across 6 suites: defaults persistence, reload suppression, workspace transactions, background agent service, command arguments, and button presentation
• The build log still reports existing SwiftLint plugin failures from CodeEditSourceEditor and CodeEditTextView dependencies; the selected AgentHub tests completed successfully and xcodebuild exited 0.