fix(split): enforce deterministic inactive split-panel fallback

[robkam]

Summary

• enforce deterministic inactive-panel fallback behavior for mirrored split-tree updates
• add focused regressions for mirrored collapse, delete invalidation, and non-invalidating add flows
• harden AI workflow autonomy rules to require immediate worker-completion handling with proactive polling

Changes

• src/ui/dir_ops.c
  • preserve inactive selection identity by path where still valid
  • resolve invalidated inactive selection with deterministic order: nearest visible ancestor, next sibling, previous sibling, root
• tests/test_panel_isolation.py
  • add mirrored delete fallback regression coverage
  • add mirrored add non-invalidation regression coverage
• docs/ai/WORKFLOW.md, .ai/shared.md
  • require proactive polling/waiting of active workers
  • require immediate completion handling sequence (read report, validate, close worker, proceed, update maintainer)

Verification

• source .venv/bin/activate && pytest tests/test_panel_isolation.py -k "mirrored or inactive_selection or dotfiles_toggle"
• source .venv/bin/activate && pytest tests/test_panel_isolation.py
• make clean && make