feat(ssh): add ssh/scp/sftp builtins with russh transport

[chaliy]

Summary

• Add SSH support as opt-in feature (ssh), following the same pattern as git and http_client
• SshHandler trait for pluggable transport, default RusshHandler backed by russh 0.52
• SshAllowlist with glob patterns (*.supabase.co) and port restrictions (default-deny)
• SshConfig builder: timeouts, response limits, session limits, default user/key/password
• ssh builtin: remote command exec, heredoc, shell sessions (ssh supabase.sh)
• scp builtin: upload/download between VFS and remote hosts
• sftp builtin: non-interactive put/get/ls via heredoc/pipe
• Auth: none (public services), pubkey, or password — tried in that order
• Example: ssh supabase.sh — no credentials needed
• 33 integration tests via mock handler + 2 real connection tests
• Shell injection fix (TM-SSH-008): all remote paths are shell-escaped

Test plan

• 33 builtin integration tests (ssh_builtin_tests.rs) covering: basic exec, user@host, shell sessions, heredoc, blocked hosts, wildcards, port flags, identity flags, pipe output, variable capture, SCP upload/download/errors, SFTP put/get/ls/errors, not-configured errors
• 2 real connection tests (ssh_supabase_tests.rs): ssh supabase.sh connect + allowlist rejection
• 13 allowlist unit tests, 3 config tests, 6 client tests
• All 1979+ existing lib tests pass with --features ssh
• cargo fmt --check clean
• cargo clippy --features ssh -- -D warnings clean
• CI: example + integration tests run in Examples job