* fix: guard against empty API response choices to prevent panic

Add ErrEmptyResponse sentinel error and bounds checking for
resp.Choices[0]
in both streaming and non-streaming API response handlers. Non-streaming
path
returns an error for empty choices array, while streaming path uses
continue
to skip empty chunks (which may occur as heartbeats).

Prevents index out of bounds panics when OpenAI API returns responses
with no
choices.

* fix: sanitize shell commands to prevent display manipulation

Add SanitizeCommand function to remove:
- ANSI escape sequences (CSI and OSC)
- Unicode bidirectional override characters
- Zero-width characters (U+200B-D, U+FEFF)
- Non-printable control characters (preserving newlines and tabs)

Apply sanitization in ExecuteCommandWithConfirmation before displaying
the
command to the user and executing it. This ensures what-you-see-is-what-
executes (WYSIWYG) and prevents display-based command injection attacks
where malicious Unicode or ANSI sequences could hide dangerous commands.

* fix: exit with non-zero code on panic recovery

Add r.exit(1) call in the panic recovery handler. Previously, panics
were
logged but the program would continue executing and exit with code 0,
masking critical failures. Now ensures that any panic results in a
non-zero
exit code, providing proper error signaling to the calling process.

* fix: add Unwrap method to exitError for error chain traversal

Implement Unwrap() method on exitError to enable proper error chain
traversal
with errors.Is() and errors.As(). This allows callers to inspect wrapped
errors and maintains Go 1.13+ error handling best practices.

* fix: check scanner error after reading chat session file

Add scanner.Err() check after the scanning loop in GetSession.
Previously,
scanner errors (such as lines exceeding bufio.MaxScanTokenSize) were
silently
ignored, potentially returning incomplete or corrupt session data. Now
properly
propagates scanner errors to the caller.

Follows the same pattern used in pkg/fs/fs.go for consistent error
handling.

* style: remove trailing newlines and fix comment alignment

- Remove trailing empty lines in api_test.go and shell_test.go
- Align comment spacing in shell.go for consistency