Handle ParseError in CLI commands for malformed input

[dithesh12]

Fixes #10

Summary

This PR handles ParseError in the CLI layer when processing malformed or truncated input.

Changes

• Catch ParseError in parse_command
• Catch ParseError in validate_command
• Display a user-facing error message instead of exposing a full traceback
• Return a non-zero exit code on failure

Validation

Malformed input:

python -m sigflow parse broken.sgf

Output:

error: truncated input

Malformed input:

python -m sigflow validate broken.sgf

Output:

error: truncated input

Valid input continues to behave as before:

python -m sigflow parse samples/valid_telemetry.sgf

This keeps parser behavior unchanged and scopes the fix to the CLI layer only, as discussed in the issue.