[deploy-plugin DPB-12] Host: CLI mount-children contribution contract

[rickylabs]

Part of #892 (RFC #891; corpus .llm/runs/plan-deploy-plugin--seed/ r5).

CliCommandContribution { mount, id, loader, export } + withCliCommands builder verb + contribution abstract + merger/verifier; host-owned reserved mounts; duplicate (mount,id) fails before parsing naming both owners. Anti-scope: no shadowing of top-level commands, no plugin-specific exceptions (DP-4 §5, SF-4).

Acceptance

• gate: duplicate and reserved-name collisions fail pre-parse with both owners named
• gate: verify-plugin gains cliCommands expectations

Metadata

Dependencies: none
Delivery shape: @netscript/plugin contribution contract