feat: prompt before launching mobile agent profiles

[leynier]

Summary

Mobile New Tab now opens a prompt dialog before spawning an opted-in Agent Profile, matching desktop #731.

• Skip launches with an empty prompt.
• Start Agent requires a non-empty prompt (typed, dictated, or an attached host path).
• Reuses the existing mobile dictation control and Add Attachment sources (photo library, files, workspace file).
• Selects the new tab after a successful launch.
• showInNewTabMenu still gates the menu. Profiles that are not opted in are not launched.

No protocol version bump. agentProfile.launch already accepts prompt, and agentProfilePromptLaunchV1 is already in MOBILE_HELLO_CAPABILITIES.

Validation

• flutter analyze on the touched mobile files (no issues)
• flutter test test/agent_profile_launch_sheet_test.dart test/workspace_tabs_screen_test.dart test/workspace_agent_comments_test.dart

Risks

• Attachment uploads still depend on the host advertising image, file, or workspace-file capabilities. The button is hidden when none are available.
• Dictation only appears when the user has enabled it.

AGENTS.md was not updated: this is UI-only on a verb the runtime already allowlists.

[pullfrog]

Important

Dictated prompts never enable Start Agent, so Skip is the only remaining launch control and it drops the text.

Reviewed changes

Mobile New Tab now collects a starting prompt before agentProfile.launch, matching desktop #731.

• Launch sheet — New dialog with Skip, Start Agent, dictation, and capability-gated attachments that insert host paths into the prompt.
• New Tab wiring — Profile menu actions carry the full AgentProfileSummary and open the sheet instead of launching immediately; a successful launch still selects the new tab.
• Tests — Widget coverage for skip, start, close, in-dialog errors, image path insertion, and the New Tab prompt round-trip.

  ^{｜ Fix all ➔ ｜ Fix 👍s ➔ ｜ View workflow run ｜ Using Grok ｜ 𝕏}

[pullfrog]

✅ No new issues found.

Reviewed changes

The launch sheet now listens on the prompt controller so dictated and other programmatic writes enable Start Agent.

• Controller listener — _promptController.addListener updates _hasPrompt and drives _canStart, matching TerminalComposeBar and replacing the onChanged rebuild.
• Coverage — Widget test assigns controller.value the same way dictation does and asserts Start Agent becomes enabled, then launches the dictated text.

  ^{｜ View workflow run ｜ Using Grok ｜ 𝕏}