Replace temp-clone with git worktree for agent-agnostic session isolation

[itdove]

Summary

Replace DevAIFlow's temp directory cloning (temp_directory.py, ~560 lines) with native git worktree commands for investigation and ticket-creation session isolation. This is faster, uses less disk space, and works with all 8 agent backends.

Full proposal document: docs/proposals/git-worktree-isolation.md

Why git worktree, not Claude Code --worktree?

Claude Code has a --worktree flag, but DevAIFlow supports 8 agent backends (Claude, Cursor, Windsurf, Copilot, Aider, Continue, Crush, Ollama). Using git worktree add/remove directly keeps the feature agent-agnostic — no changes needed to AgentInterface.

Current state

• devflow/utils/temp_directory.py (~560 lines) — full git clone to /tmp/, complex cleanup
• Session creation: 2-10 seconds (full clone)
• Disk usage: full duplicate of repository

Proposed state

• devflow/utils/worktree.py (~100 lines) — git worktree add/remove wrappers
• Session creation: ~100ms (worktree add)
• Disk usage: shared .git directory, only working files duplicated
• Worktrees stored in .daf/worktrees/<session-name>/

Changes required

New: devflow/utils/worktree.py (~100 lines)
Remove: devflow/utils/temp_directory.py (~560 lines)
Modify:

• devflow/cli/commands/git_new_command.py — use worktree instead of temp-clone
• devflow/cli/commands/open_command.py — restore session in existing worktree
• devflow/cli/signal_handler.py — worktree cleanup on exit
• devflow/session/models.py — add worktree_path field

Net: ~460 lines removed

Edge cases

• Non-git repos: fall back to current temp-clone
• Dirty worktree on exit: prompt user to keep or remove
• Session reopen: check if worktree exists, recreate from branch if not
• Env files: .dafworktreeinclude pattern copies gitignored files into worktree

• git worktree add used instead of git clone for session isolation
• Works with all 8 agent backends (agent-agnostic)
• temp_directory.py removed
• Worktrees stored in .daf/worktrees/ at repo root
• .dafworktreeinclude copies env files into worktrees
• Clean worktree removal on session exit
• Non-git repos fall back gracefully
• All existing tests pass