CLI: add 'aimdb instance ping' subcommand

[lxsaah]

Difficulty: medium · Estimate: 3–4 hours

The CLI has instance, record, graph, watch, generate subcommands. A ping subcommand that connects to a remote AimDB instance via the existing AimX protocol and reports connectivity + RTT would be a great quick-check for users wiring up remote access. Self-contained scope.

Where to look

• tools/aimdb-cli/src/main.rs — top-level command list
• tools/aimdb-cli/src/commands/instance.rs — pattern for a new subcommand under instance
• aimdb-core/src/remote/ — protocol primitives

What to do

• Add Ping { url: String, count: Option<u32> } to the instance subcommand enum.
• Implement: open the AimX session, send a no-op request, measure RTT, repeat count times (default 1), print min/avg/max.
• Print results in both human and --json form.

Done when

• aimdb instance ping <url> prints RTT info
• Includes a test of the args parsing
• Behaves cleanly on a closed/unreachable port (clear error, non-zero exit)

We'd love help with this — comment if you'd like to take it.