fix: implement SSH host key verification to prevent MitM attacks

[devin-ai-integration]

Summary

Replaces ssh.InsecureIgnoreHostKey() in pkg/proxy/proxy.go with a fail-closed HostKeyCallback() that supports two verification modes via environment variables:

• SSH_KNOWN_HOSTS_FILE — path to an OpenSSH known_hosts file, verified via knownhosts.New()
• SSH_HOST_KEY — base64-encoded raw public key for pinning (e.g. from ssh-keyscan | awk '{print $3}')

If neither is set, DialThroughBoundary returns an error — no silent fallback to insecure mode.

// before
HostKeyCallback: ssh.InsecureIgnoreHostKey(),

// after
hostKeyCb, err := HostKeyCallback() // reads SSH_KNOWN_HOSTS_FILE or SSH_HOST_KEY
// ...
HostKeyCallback: hostKeyCb,

New file pkg/proxy/hostkey.go contains HostKeyCallback(), ErrHostKeyMismatch, and the pinned/known_hosts callbacks.

Tests added in main_test.go:

• TestHostKeyMismatch — pins wrong key, asserts ErrHostKeyMismatch
• TestHostKeyMatch — pins correct key, asserts successful connection
• TestHostKeyCallback_NeitherConfigured — asserts error when unconfigured
• TestHostKeyCallback_KnownHostsFile — writes a temp known_hosts, verifies acceptance

Fixes #13

Link to Devin session: https://app.devin.ai/sessions/2a885a94e7464a9993c6f40865502ea9

[devin-ai-integration]

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