feat: interactive permission UI — diff view + arrow-key confirmation for tool calls in ask mode

[VforVitorio]

What problem does this solve?

When the agent is in ask mode and wants to execute a tool call (e.g. write_file, run_shell), the current confirmation flow is basic and doesn't let the user review the proposed changes before approving them.

Proposed solution

Replace the current implicit execution with a rich interactive confirmation UI that lets the user review changes before they are applied.

Diff view (for write_file / file modifications)

Show the proposed change as a unified diff in a styled Rich Panel before the confirmation prompt:

┌─ write_file  src/lmcode/agent/core.py ──────────────────────────────────────┐
│  - old line A                                                                │
│  + new line A (modified)                                                     │
│  + new line B (added)                                                        │
└──────────────────────────────────────────────────────────────────────────────┘

Lines removed in ERROR red, lines added in SUCCESS green — matching the DIFF_ADD/DIFF_REMOVE color aliases already in colors.py.

Confirmation widget

Below the diff, an arrow-key-navigable inline menu styled in ACCENT violet:

  Allow this change?

  ❯ Yes
    No
    Yes — and allow this tool automatically from now on

  ┌──────────────────────────────────────────────────────┐
  │ ...or tell lmcode what to do instead                 │
  └──────────────────────────────────────────────────────┘

• ↑/↓ arrows navigate the three options.
• Enter confirms the selected option.
• Selecting the text box allows the user to type a redirect instruction (e.g. "use a different file name") which is fed back to the model as a new user turn.
• "Yes — and allow automatically" sets the tool to auto-approve for the rest of the session (equivalent to switching that specific tool to auto mode).

For run_shell

Show the exact command in a styled code block instead of a diff.

Implementation notes:

• Hook into the _wrap_tool_verbose path — intercept in ask mode before calling the real tool.
• Use prompt_toolkit for the arrow-key menu (inline, no full-screen).
• Diff generation: compare existing file content with proposed content using difflib.unified_diff.
• The "allow automatically" option could set a per-tool flag in Agent (e.g. self._auto_tools: set[str]).

Acceptance criteria:

• In ask mode, write_file and run_shell calls show the confirmation UI before executing.
• The diff is syntax-highlighted with correct add/remove colors.
• Arrow-key navigation works in the VSCode integrated terminal.
• Auto-allow state persists for the session, resets on /clear.

Alternatives considered

None considered.

[VforVitorio]

Resolved in PR #95: Interactive permission UI with native Rich prompt-toolkit integration and syntax-highlighted diff side-by-side view successfully implemented in \eat/interactive-permissions.