Migrate Ava to run as ben user with dedicated worktree

Currently Ava runs as a separate 'ava' user, which causes many issues:

• File permission problems
• Git config/hooks don't work
• Need symlinks in /usr/local/bin for tools
• Can't use ben's pi token (must use OpenRouter, more expensive)
• Separate /run/secrets for API keys

New approach: 1. Create dedicated git worktree: /home/ben/omni/ava 2. Run Ava systemd service as ben user 3. Use ben's pi auth tokens (Claude Code API, cheaper than OpenRouter) 4. Ava works in its worktree, ben works in /home/ben/omni/live 5. Both share same .git, same branches, same environment

Benefits:

• No permission issues (same UID)
• direnv/nix just works
• Git hooks work
• Direct Anthropic API (no OpenRouter markup)
• Simpler deployment