Fix sandbox git checkout failure with beads-aware stashing

[landovsky]

Summary

Replaces the broken beads sync approach (commit 58ed160) with a proper stashing solution that prevents sandbox git checkout failures.

Problem

The previous fix attempted to auto-run bd sync inside the container, but failed because:

1. Wrong grep pattern - bd sync output didn't match expected strings
2. bd sync doesn't commit - it only exports database to JSONL, user must commit separately
3. Wrong approach - sandbox is ephemeral and should use clean remote state, not preserve local changes

This caused the sandbox to still fail with:

[sandbox] Repository still has uncommitted changes. Please commit or stash:
 M .beads/issues.jsonl

Solution

New beads-aware stashing approach:

• Automatically stashes uncommitted changes with descriptive timestamp
• Differentiates messaging between beads-only vs mixed changes
• Always shows what's being stashed and how to recover
• Falls back to force clean if stash fails
• Proceeds with clean remote state (appropriate for ephemeral test environment)

Testing

• ✅ Tested locally with claude-sandbox local
• ✅ Verified clean repository works normally
• ✅ Verified dirty beads files are stashed with appropriate messaging
• ⏳ Remote execution will be tested after build

🤖 Generated with Claude Code