fix(cli): use --name flag in gateway destroy help messages

[drew]

Summary

Fix invalid gateway destroy commands in error messages and recovery hints. The CLI defines name as a --name flag, but all help messages were showing it as a positional argument, causing unexpected argument errors when users copied the suggested commands.

Changes

• crates/openshell-bootstrap/src/errors.rs — Fix 7 occurrences of openshell gateway destroy {name} to openshell gateway destroy --name {name}
• crates/openshell-cli/src/run.rs — Fix the "gateway not reachable" recovery hint
• crates/openshell-bootstrap/src/lib.rs — Fix the "gateway already exists" error message

Testing

• mise run pre-commit passes
• cargo check passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)