fix: remove invalid Ctrl+Shift+C keybinding

[nomuvan]

Summary

• Remove the @kb.add('c-S-c') prompt_toolkit binding that raises ValueError: Invalid key: c-S-c during CLI startup.
• Leave Ctrl+Shift+C/Cmd+C handling to terminal emulators.
• Add regression coverage that validates literal CLI keybinding declarations against prompt_toolkit.

Test Plan

• python -m pytest tests/cli/test_cli_keybindings.py tests/cli/test_cli_extension_hooks.py -q
• hermes --version
• hermes chat -q 'pingとだけ返して' --quiet
• Interactive hermes launch in tmux

[teknium1]

Duplicate — the underlying bug was fixed on main before your PR landed.

• PR #19895 by @nftpoetrist added the try/except guard (merged 2026-05-04 21:45 UTC, commit 429b8ec).
• PR #19919 followed up by removing the dead handler entirely.

Closing issue #19903 too. Thanks for jumping on it — appreciate the quick turnaround even if we only needed one of the fixes.