feat(worktree): add setup script support for post-creation automation

[Clarity-89]

Summary

• Add convention-based .agent-deck/worktree-setup.sh that runs automatically after worktree creation
• Allows repos to copy gitignored config files (.env, .mcp.json, etc.) into new worktrees
• CLI paths stream script stdout/stderr in real-time for progress feedback
• TUI paths buffer output and log warnings on failure
• Setup script failure is non-fatal — worktree is still created with a warning

How it works

1. After CreateWorktree succeeds, checks for .agent-deck/worktree-setup.sh in the source repo
2. If found, runs it via sh -e with AGENT_DECK_REPO_ROOT and AGENT_DECK_WORKTREE_PATH env vars
3. 60-second timeout prevents hanging scripts from blocking session creation

Example .agent-deck/worktree-setup.sh:

#!/bin/sh
for f in .env .env.local .mcp.json; do
    [ -f "$AGENT_DECK_REPO_ROOT/$f" ] && cp "$AGENT_DECK_REPO_ROOT/$f" "$AGENT_DECK_WORKTREE_PATH/$f"
done

Test plan

• 8 unit tests covering find, run (success/failure/timeout), and the combined wrapper
• Manual: create worktree session in repo with setup script — verify files copied
• Manual: create worktree session in repo without setup script — no change in behavior
• Manual: make setup script fail — verify worktree still created with warning