Add repo-scoped filtering to runbox ps/history

[proboscis]

Problem

Currently runbox ps and runbox history list ALL runs from global storage (~/.local/share/runbox/runs/), regardless of which repo the user is in.

For users working on multiple projects, this creates noise - runs from unrelated projects are mixed together.

Design Decisions

1. Default behavior: Repo-scoped (BREAKING CHANGE)

# In a git repo (main or worktree):
runbox ps          # filters to current repo (NEW default)
runbox history     # filters to current repo (NEW default)

# Escape hatch for global view:
runbox ps --global
runbox history --global

2. Worktree handling

When in a worktree, resolve to the main repo's origin URL. This groups all experiments across worktrees together.

main-repo/
├── worktree-exp-v1/  → same repo URL
├── worktree-exp-v2/  → same repo URL  
└── worktree-exp-v3/  → same repo URL

# In ANY of these dirs:
runbox ps  → shows runs from ALL worktrees of this repo

3. Outside git repo: Error

$ cd /tmp
$ runbox ps
Error: Not in a git repository. Use --global to see all runs.

$ runbox ps --global
# Works, shows all runs

Implementation Notes

• Run struct already stores code_state.repo_url
• Use GitContext::from_current_dir() to get repo URL (works in worktrees too)
• Add --global flag to ps and history commands
• Filter: run.code_state.repo_url == current_repo_url
• URL normalization: extract org/repo portion for comparison (handles SSH vs HTTPS)

Acceptance Criteria

• runbox ps in git repo filters to current repo by default
• runbox ps in worktree filters to main repo
• runbox ps --global shows all runs regardless of location
• runbox ps outside git repo errors with helpful message
• runbox history same behavior as ps
• URL matching handles SSH/HTTPS variants of same repo