feat(ssh): add --instance option to target a running instance by ID

[mehdi653]

Summary

When several instances are running, clever ssh currently requires an interactive prompt and fails in non-interactive mode. This adds --instance instance-id so users and scripts can target a specific running instance without a prompt.

Instance IDs are used rather than instance numbers, which are not stable across deployments (per hsablonniere feedback on #1067). Support for number / any / all selection is left for a follow-up (judu).

Fixes #1067

Change

• Add --instance instance-id option to clever ssh
• When set, select that running instance and skip the interactive prompt
• When the ID does not match a running instance of the app, fail with a clear error
• Update ssh docs and generated skill reference

Spec notes

• Selection key is the Clever Cloud instance ID (not the ephemeral instance number)
• Existing behavior is unchanged when --instance is omitted (single instance auto-select, multi-instance TTY prompt, non-TTY failure)
• Works with -c / --command for non-interactive one-shot commands

Test plan

• run validate on this branch
• valid instance id connects without prompt
• bogus instance id fails with a clear error
• multi-instance non-TTY with option succeeds
• multi-instance non-TTY without option still fails as before

E2E results (2026-08-13)

Tested against a real Clever Cloud Node app with the local CLI from this PR checkout (HOME/.local/bin clever, which exposes --instance).

App: ssh-instance-e2e-1123 (app_99596987-df36-4925-8bab-26f50b2e1ba5), scaled to 2 instances, dedicated ed25519 key registered on the account.

1. Valid instance id, agent disabled, remote echo:
   -> OK, exit 0, no prompt
2. Bogus instance id:
   -> clear error: Instance bogus is not a running instance of this application, exit 1
3. Non-TTY multi-instance WITH --instance (stdin closed) on the second instance id:
   -> OK, exit 0
4. Non-TTY multi-instance WITHOUT --instance:
   -> clear error: Multiple instances are running. Cannot select in non-interactive mode., exit 1

Also: validate passed on 5898785 (lint, prettier, typecheck, docs:check). Working tree clean; no docs rewrite and no force-push needed.

Test app and temporary SSH key were deleted after the run.