Add internal code mode module for programmatic notebook control (marimo-team#8670)

This introduces `marimo._code_mode`, an internal agent-only API that
gives programmatic access to a running marimo notebook. The motivating
use case is letting agents (e.g. from a scratchpad) insert, delete,
replace, and reorder cells without going through the frontend UI. e.g.,

```python
import marimo._code_mode as cm

async with cm.get_context() as ctx:
    # Install packages (queued, installed before cell ops)
    ctx.install_packages("pandas", "polars>=0.20")

    # Cell ops (appends at end by default)
    cid = ctx.create_cell("import pandas as pd")
    ctx.create_cell("df = pd.DataFrame()", after=cid)
    ctx.create_cell("setup()", before=cid, hide_code=True, disabled=True)
    
    ctx.update_cell("my_cell", code="x = 42")
    ctx.update_cell("other", hide_code=False, disabled=True)
    ctx.delete_cell("old_cell")
    ctx.move_cell("my_cell", after="other_cell")

    # Set UI element values (batched)
    ctx.set_ui_value(slider, 10)

# Dry-run compile check is on by default; disable with:
async with cm.get_context(check=False) as ctx:
    ...
```

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: manzt

marimo/_code_mode/__init__.py

@@ -0,0 +1,45 @@
+# Copyright 2026 Marimo. All rights reserved.
+"""Code mode: programmatic notebook editing via async context manager.
+
+.. warning::
+
+    **Internal, agent-only API.** Not part of marimo's public API.
+    No versioning guarantees. May change or be removed without notice.
+
+Usage::
+
+    import marimo._code_mode as cm
+
+    async with cm.get_context() as ctx:
+        # Create cells (appends at end by default)
+        cid = ctx.create_cell("x = 1")
+        ctx.create_cell("y = x + 1", after=cid)
+
+        # Update cells by ID or name
+        ctx.edit_cell("my_cell", code="z = 42")
+
+        # Delete cells
+        ctx.delete_cell("old_cell")
+
+        # Move cells
+        ctx.move_cell("my_cell", after="other_cell")
+
+    # Read cells (works outside the context manager too)
+    ctx = cm.get_context()
+    ctx.cells[0]  # by index
+    ctx.cells["my_cell"]  # by name
+"""
+
+from __future__ import annotations
+
+from marimo._code_mode._context import (
+    AsyncCodeModeContext,
+    NotebookCellData,
+    get_context,
+)
+
+__all__ = [
+    "AsyncCodeModeContext",
+    "NotebookCellData",
+    "get_context",
+]