Make `nemoclaw onboard` robust when rerun after a failed install instead of requiring `uninstall.sh` as a workaround

[kjw3]

Summary

When installation or onboarding fails, the known workaround is to run uninstall.sh before trying again. This recovery path is not documented in product output and suggests the setup flow is not idempotent.

Problem

Users should not need to discover from external notes that a full uninstall is required to recover from a transient namespace or gateway failure.

Expected behavior

Onboarding should either be safely rerunnable or should print an explicit cleanup step when the environment cannot be reused.

Acceptance criteria

• Failed onboarding attempts can be retried without a full manual uninstall in common cases.
• If a retry requires cleanup, the CLI prints the exact remediation command.
• The troubleshooting docs include the current rerun failure modes.
• A test covers rerunning onboarding after an interrupted or failed setup.

[kjw3]

PR #95 should stay scoped to issue #17: failed gateway startup leaving stale Docker volume state that breaks reruns. That closes one concrete rerun/idempotency failure under this broader issue, but it does not fully resolve the wider onboard-rerun robustness work tracked here.

[ericksoa]

The common failure cases here have been addressed across several merged PRs:

• fix: harden installer and onboard resiliency #961 — hardened installer and onboard resiliency
• fix: harden inference onboarding auth and validation recovery #1136 — auth and validation recovery during onboarding
• fix(cli): prompt before reusing existing sandbox on re-onboard #1527 — --recreate-sandbox flag and interactive prompt when a sandbox already exists on re-onboard
• fix: clear onboard session when stale sandbox is removed (#1316) #1555 — stale onboard session cleanup on sandbox removal

Re-running nemoclaw onboard after a failure no longer requires a full uninstall.sh. Remaining work on making onboard truly resumable mid-flow (pick up where it left off) is tracked in #446. Closing this one.