feat(interactive): worker launch-spec for embedded ConPTY sessions

Adds InteractiveLaunchSpecService + GetInteractiveLaunchSpec hub method that
prepares a task worktree (session-skills seeding, run env) and returns a
LaunchSpec {cwd,exe,args,env} for a UI-hosted ConPTY terminal. Reuses
ISessionSkillSeeder, TaskRunner.UnionSkillNames, and WindowsTerminalLauncher
resume-arg/resolve logic. Guards mirror ResumeTaskInTerminal; a never-run task
yields a fresh-start spec instead of an error.

src/ClaudeDo.Worker/Runner/Interfaces/IInteractiveLaunchSpecService.cs

@@ -0,0 +1,10 @@
+namespace ClaudeDo.Worker.Runner;
+
+public interface IInteractiveLaunchSpecService
+{
+    /// <summary>Builds a LaunchSpec for opening an interactive session in an existing task's
+    /// worktree. Throws KeyNotFoundException if the task doesn't exist, InvalidOperationException
+    /// if it's Running/Queued or has no usable worktree. A task that has never run (no persisted
+    /// SessionId) is not an error -- the spec omits --resume for a fresh start.</summary>
+    Task<LaunchSpec> BuildForTaskAsync(string taskId, CancellationToken ct);
+}