feat: Add stacked branches / stacked PRs as first-class worktree feature

[Cheezeiii365]

Summary

Add first-class support for stacked branches (branch-from-branch development) inside aIDE's worktree system. This lets users keep shipping while PRs are under review by branching from in-flight feature branches instead of waiting for them to merge to main.

Background

This is a well-established workflow supported by tools like Graphite, ghstack, Git Town, and Sapling. The core idea:

1. Branch B from branch A (your PR branch), not from main
2. Open PR(B) with base=A
3. Keep shipping while A is under review
4. After A merges: rebase B onto main, resolve conflicts once, continue

aIDE already has strong primitives for this via worktrees and base-branch creation in worktreeManager.ts and CreateWorktreeModal.tsx.

Proposed Feature (v1)

Core Capabilities

• "Create Child Branch" action from a worktree item
• Track parent/child relationships in local app state (not committed)
• "Sync Stack" command:
  • Fetch + detect merged parents
  • Auto-rebase children in order
  • Pause on conflict with guided resolve UI
• Stack graph in Worktree panel (A → B → C) with statuses:
  • Ahead/behind, needs rebase, conflicted, merged parent
• Optional GitHub integration: create stacked PRs and auto-retarget base branches

Important Caveats

• This rewrites commit history (rebases) — educate users on force-push-to-feature-branch
• Never auto-rebase without opt-in + dry-run preview
• Conflict handling UX is the make-or-break piece

File Map

File	Action	Description
packages/shared/src/index.ts	edit	Add branch-stack types (node id, parent branch, sync status, conflict state) + IPC channels (STACK_LIST, STACK_CREATE_CHILD, STACK_SYNC, STACK_RETARGET)
packages/main/src/workspace/worktreeManager.ts	edit	Add child-branch creation handler (base defaults to active branch) + stack-sync orchestration (ordered rebase pipeline, dry-run mode, conflict reporting)
packages/main/src/git/branchStack.ts	new	Pure git operations: merge-base, rebase, ancestry checks, merged detection — separated from IPC/UI concerns
packages/renderer/src/components/WorktreePanel/WorktreePanel.tsx	edit	Render parent/child indicators, "Create Child Branch" action, sync status badges, conflict CTAs
packages/renderer/src/components/WorktreePanel/CreateWorktreeModal.tsx	edit	Add "Create as child of current branch" shortcut preset, preselect base branch to active worktree branch
packages/renderer/src/hooks/useWorktrees.ts	edit	Subscribe to stack status updates, expose sync actions to UI

Practical Workflow (Manual, Available Now)

1. Build feature A on feature/A
2. While A in review, create feature/B from feature/A in a separate worktree
3. Open PR B with base branch = feature/A
4. Keep bugfixing A; periodically rebase feature/B onto updated feature/A
5. When A merges, rebase B onto main, retarget PR B to main