feat: add main-repo worktree mode for topics

[ByteMirror]

Summary

• Replaces SharedWorktree bool on Topic with a TopicWorktreeMode string enum (per_instance, shared, main_repo)
• Adds a third topic mode: Main repo (no worktree) — instances run directly in the repository directory without creating a new git branch or worktree
• Swaps the old Y/N "Create shared worktree?" confirmation with a 3-item picker at topic creation time

What changed

Layer	Change
session/topic.go	TopicWorktreeMode enum + IsSharedWorktree() / IsMainRepo() helpers
session/topic_storage.go	New worktree_mode JSON field; legacy shared_worktree bool is auto-migrated on read
session/instance.go	mainRepo bool field + GetWorkingPath() (returns worktree path or repo path)
session/instance_lifecycle.go	StartInMainRepo() method; Pause()/Resume() guarded against nil worktree
app/app_input.go	ConfirmationOverlay → PickerOverlay with 3 options; MainRepo dispatch branch
app/app_state.go	5 GetGitWorktree() callers replaced with GetWorkingPath()
app/app_brain.go	MainRepo dispatch branch for brain-spawned instances
app/app.go	View: stateNewTopicConfirm now renders pickerOverlay (not confirmationOverlay)

UX

When creating a topic (T), instead of a Y/N dialog the user sees:

Worktree mode for 'my-topic'
  ▸ Per-instance worktrees
    Shared worktree
    Main repo (no worktree)

Backward compatibility

Existing topics stored with "shared_worktree": true/false are automatically migrated to the new enum on first read. No manual migration required.

Test plan

• Create a topic, verify picker shows all 3 options
• Select Main repo — instance starts, working directory is the repo root (not a worktree)
• Select Per-instance worktrees — existing behavior unchanged
• Select Shared worktree — existing behavior unchanged
• Restart hivemind — topics survive restart with correct mode
• Old topics (no worktree_mode in JSON) load correctly as per-instance or shared