feat: major /rollback improvements — enabled by default, diff preview, file-level restore, conversation undo, terminal checkpoints

Checkpoint & rollback upgrades:

1. Enabled by default — checkpoints are now on for all new sessions.
   Zero cost when no file-mutating tools fire. Disable with
   checkpoints.enabled: false in config.yaml.

2. Diff preview — /rollback diff <N> shows a git diff between the
   checkpoint and current working tree before committing to a restore.

3. File-level restore — /rollback <N> <file> restores a single file
   from a checkpoint instead of the entire directory.

4. Conversation undo on rollback — when restoring files, the last
   chat turn is automatically undone so the agent's context matches
   the restored filesystem state.

5. Terminal command checkpoints — destructive terminal commands (rm,
   mv, sed -i, truncate, git reset/clean, output redirects) now
   trigger automatic checkpoints before execution. Previously only
   write_file and patch were covered.

6. Change summary in listing — /rollback now shows file count and
   +insertions/-deletions for each checkpoint.

7. Fixed dead code — removed duplicate _run_git call in
   list_checkpoints with nonsensical --all if False condition.

8. Updated help text — /rollback with no args now shows available
   subcommands (diff, file-level restore).

run_agent.py

@@ -205,6 +205,33 @@ _NEVER_PARALLEL_TOOLS = frozenset({"clarify"})
 # Maximum number of concurrent worker threads for parallel tool execution.
 _MAX_TOOL_WORKERS = 8
 
+# Patterns that indicate a terminal command may modify/delete files.
+_DESTRUCTIVE_PATTERNS = re.compile(
+    r"""(?:^|\s|&&|\|\||;|`)(?:
+        rm\s|rmdir\s|
+        mv\s|
+        sed\s+-i|
+        truncate\s|
+        dd\s|
+        shred\s|
+        git\s+(?:reset|clean|checkout)\s
+    )""",
+    re.VERBOSE,
+)
+# Output redirects that overwrite files (> but not >>)
+_REDIRECT_OVERWRITE = re.compile(r'[^>]>[^>]|^>[^>]')
+
+
+def _is_destructive_command(cmd: str) -> bool:
+    """Heuristic: does this terminal command look like it modifies/deletes files?"""
+    if not cmd:
+        return False
+    if _DESTRUCTIVE_PATTERNS.search(cmd):
+        return True
+    if _REDIRECT_OVERWRITE.search(cmd):
+        return True
+    return False
+
 
 def _inject_honcho_turn_context(content, turn_context: str):
     """Append Honcho recall to the current-turn user message without mutating history.
@@ -3842,6 +3869,18 @@ class AIAgent:
                 except Exception:
                     pass
 
+            # Checkpoint before destructive terminal commands
+            if function_name == "terminal" and self._checkpoint_mgr.enabled:
+                try:
+                    cmd = function_args.get("command", "")
+                    if _is_destructive_command(cmd):
+                        cwd = function_args.get("workdir") or os.getenv("TERMINAL_CWD", os.getcwd())
+                        self._checkpoint_mgr.ensure_checkpoint(
+                            cwd, f"before terminal: {cmd[:60]}"
+                        )
+                except Exception:
+                    pass
+
             parsed_calls.append((tool_call, function_name, function_args))
 
         # ── Logging / callbacks ──────────────────────────────────────────
@@ -4035,6 +4074,18 @@ class AIAgent:
                 except Exception:
                     pass  # never block tool execution
 
+            # Checkpoint before destructive terminal commands
+            if function_name == "terminal" and self._checkpoint_mgr.enabled:
+                try:
+                    cmd = function_args.get("command", "")
+                    if _is_destructive_command(cmd):
+                        cwd = function_args.get("workdir") or os.getenv("TERMINAL_CWD", os.getcwd())
+                        self._checkpoint_mgr.ensure_checkpoint(
+                            cwd, f"before terminal: {cmd[:60]}"
+                        )
+                except Exception:
+                    pass  # never block tool execution
+
             tool_start_time = time.time()
 
             if function_name == "todo":