feat: work item launch with mode selection and default prompt generation

[cirvine-MSFT]

User Feedback

When opening a Work Item into an EditLess session, users want to choose session mode (agent/plan/autopilot), start from a generated default prompt, and optionally edit before launch.

Problem

Current flow requires extra manual steps when launching a session from a work item. No way to:

• Choose the session mode (agent / plan / autopilot)
• Start from a generated default prompt based on the work item
• Edit the prompt before the session opens

Why It Matters

• Users want a fast "one click to productive session" workflow
• Especially useful for repeatable or structured work items
• Reduces friction for the most common EditLess workflow

Suggested Enhancement

• Quick pick to select mode before launch
• Generate a default prompt from work item title + description
• Show an input box with the generated prompt, editable before launch
• Optionally: configurable "launch command" or template per work item type

Scope

• src/commands/work-item-launcher.ts — launch flow
• src/commands/work-item-commands.ts — command handlers
• May overlap with feat: pass work item context to agent sessions launched via 'Launch with Agent' #476 (work item context passthrough)