[FEATURE] Make chat input cancel key configurable instead of forcing <Esc> to cancel

## Feature Description

Allow customizing which key cancels the chat input in claudecode.nvim, rather than hardcoding `` to cancel. Ideally, `` could be used to leave insert mode and enter normal mode in the chat input buffer.

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## Use Case

Currently, pressing `` in the chat input window cancels the request entirely, which makes it impossible to switch from insert mode to normal mode in the input. Many Neovim users expect `` to leave insert mode and allow normal-mode editing operations (like moving the cursor, deleting words, etc.) inside the buffer.

This feature would make the chat input behave like any other Neovim buffer and avoid accidental cancellations when trying to exit insert mode.

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## Proposed Solution

Add a plugin option such as:

```lua
opts = {
cancel_key = "",
}
```

Then use this key to trigger cancellation instead of always binding ``.

Alternatively, allow `` to switch to normal mode, and only cancel the chat if the user explicitly types another command to close it.

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## Alternatives Considered

* Patching the plugin manually to change the `` mapping.
* Avoiding insert mode entirely in the chat input, but this is impractical for normal usage.
* Using a different plugin, but claudecode.nvim fits my workflow best.

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## Additional Context

Other AI plugins like Copilot Chat and ChatGPT Neovim allow normal-mode editing inside their input buffers, which improves usability. Bringing similar behavior to claudecode.nvim would make the chat experience smoother for Vim users.

Thanks for considering this!

Start by locating the chat input implementation and its hardcoded cancellation mapping. Add a configurable cancellation key, then verify that can leave insert mode while the configured key cancels the request; the issue does not name a specific file or test.

Written by the indexing model from the issue text.