feat: add st track, create, log, up, down and stack-wide submit

[siner308]

Summary

• Add st track / st untrack for explicit branch tracking via git config (branch.<name>.staqd-parent)
• Add st create for branch creation with auto-tracking
• Add st log for tracked stack tree visualization
• Add st up / st down for stack navigation
• Update st submit to create PRs for the entire tracked chain (root → leaf), not just the current branch
• Add track helpers to cli/git.mjs (getTrackedParent, setTrackedParent, unsetTrackedParent, listTrackedBranches)

Motivation

Previously st submit only created a PR for the current branch. With stacked workflows (main → a → b → c), users had to run st submit on each branch individually. Now, st track registers the parent relationship in git config, and st submit walks the chain to create all missing PRs at once — similar to Graphite's gt track + gt submit.

New Commands

Command	Description
st track	Register current branch (auto-detect or --parent)
st track --list	Show all tracked branches
st untrack	Remove current branch from tracking
st create <name>	Create branch + auto-track
st log	Tree visualization of tracked stack
st up	Checkout child branch
st down	Checkout parent branch

Design Decisions

• Git config storage (branch.<name>.staqd-parent): Same approach as Graphite. No external DB, survives branch operations, portable.
• Parent validation: Parent must be default branch or a tracked branch whose chain reaches default branch. Prevents orphan PRs.
• Backward compatible: st submit without any tracked branches falls back to the existing detectBase behavior.

Test plan

• st track --parent=main on a branch → verify git config set
• st track --list shows tracked branches
• st untrack removes tracking
• st log shows tree with current branch highlighted
• st create <name> creates branch and auto-tracks
• st up / st down navigate the stack
• st submit --dry-run on a tracked stack shows PRs to create for entire chain
• st submit creates PRs for all unsubmitted branches in chain
• Untracked branch + st submit falls back to existing behavior

---

Generated with Claude Code