feat(ssh): forward Ctrl+C (SIGINT) to remote command

[risu729]

Problem

biwa does not install a handler for local SIGINT (Ctrl+C). The OS default terminates the client immediately, so the SSH session drops without telling the remote process to stop. Users expect Ctrl+C to interrupt the remote command (similar to OpenSSH), not only kill the local CLI.

Current behavior

• Ctrl+C delivers SIGINT to biwa; the process exits with the usual shell convention (often exit code 130).
• The SSH connection is torn down abruptly; there is no application-level forwarding of the interrupt to the remote session.

Desired behavior

• While a remote command is running (especially with an interactive PTY / stdin forwarding), Ctrl+C should interrupt the remote process (or send the equivalent on the channel), not only exit the local client.
• Graceful shutdown of the local client can remain a separate concern (e.g. second Ctrl+C, or explicit disconnect), but the primary interrupt should reach the remote.

Notes / implementation sketch

• Likely requires handling tokio::signal::ctrl_c() (or equivalent) and mapping it to SSH semantics: e.g. send an interrupt character on the PTY stream and/or a channel signal request, depending on session type and what russh supports.
• Must cover both PTY and non-PTY execution paths in src/ssh/exec.rs (and related client code) where behavior may differ.

Related

Remote exit status mapping for SIGINT on the server side already exists in src/ssh/client/execute.rs (ExitSignal → synthetic 130); this issue is about local Ctrl+C reaching the remote, not interpreting remote exit signals.