[FEATURE] Support custom wrapper commands (like shell functions/aliases) for terminal_cmd

## Feature Description

Support for using **custom wrapper commands** (e.g. shell functions or aliases) as `terminal_cmd` in `claudecode.nvim`.
Currently, `terminal_cmd` only works with executables available in the shell PATH. When using a shell function (e.g. `cl`), it fails with exit code 127.

## Use Case

I created a custom shell function `cl` that wraps `claude` with additional custom prompts.
I’d like to use this wrapper command with `claudecode.nvim` so that every request automatically follows my predefined prompt rules.
This would make it easier for users to customize their workflows without modifying the plugin code.

## Proposed Solution

Allow `terminal_cmd` to support shell functions or aliases in addition to executables.
For example, running the command through a shell (e.g. `bash -i -c "cl"`) so that user-defined functions are available.

## Alternatives Considered

- Directly using the `claude` binary instead of the wrapper (works, but loses my custom prompt logic).
- Writing a separate executable script (`~/bin/cl`) instead of a shell function (possible, but less convenient for quick customization).

## Additional Context

Error message when attempting to use a shell function as `terminal_cmd`:

[ClaudeCode] [terminal] [ERROR] Claude exited with code 127.
Check for any errors.

```lua
Example config:

{
"coder/claudecode.nvim",
dependencies = { "folke/snacks.nvim" },
opts = {
terminal_cmd = "cl",
terminal = {
snacks_win_opts = {
...
},
},
},
}

```

Beginne bei der terminal_cmd-Behandlung des Plugins und reproduziere den gemeldeten Exit-Code 127 mit der Beispiel-Shell-Funktion cl. Ermittle, wie die Befehlsausführung derzeit PATH-Executables behandelt, und verifiziere anschließend, dass eine unterstützte Wrapper-Funktion oder ein Alias funktioniert, ohne direkte ausführbare Befehle zu beeinträchtigen.

Vom Indexierungsmodell aus dem Issue-Text verfasst.