Add workspace field to session JSON

[christinoleo]

Add a workspace field to session JSON files to support grouping sessions across distributed agents.

Details

• Hook detects workspace (git root, explicit config, or cwd-based)
• Field is optional for backward compatibility
• UI uses workspace for grouping instead of path-prefix logic

Acceptance Criteria

• Hook writes workspace field to JSON
• Workspace detection: try git root, fall back to config/cwd
• UI groups by workspace when available
• Backward compatible with existing JSON files

Parent epic: #1

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Migrated from beads claude-watch-d7j.