Prevent agents from escaping worktree via prompt constraint

Root cause found via agent logs: Claude Code agents cd out of their
worktree into the main repo when they encounter errors (e.g., npm test
fails in worktree). This abandons the worktree, and subsequent git
operations in the main repo context can destroy worktree metadata.

Fix: append worktree constraint to agent prompt instructing it to stay
in the current directory and not run git worktree/init/thinktank commands.

Evidence from agent-1 logs:
  cd C:/Users/silon/Documents/repos/thinktank && npm run lint

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: unknown

src/runners/claude-code.ts

@@ -38,9 +38,15 @@ export const claudeCodeRunner: Runner = {
       let error = "";
       let settled = false;
 
+      // Append worktree constraint to prompt to prevent agents from escaping
+      const constrainedPrompt =
+        `${opts.prompt}\n\nIMPORTANT: Work ONLY in the current directory (${opts.worktreePath}). ` +
+        "Do NOT cd to other directories. Do NOT run git worktree, git init, or thinktank commands. " +
+        "All your changes must be made in the current working directory.";
+
       const args = [
         "-p",
-        opts.prompt,
+        constrainedPrompt,
         "--output-format",
         "text",
         "--model",