A parent funds a child forge's crew: a $ cap per child on the parent's own credential, metered, shown on both forges #10484

The owner, 2026-11-10, about scottishbob (a friend's hosted child): give it "like $200 in creds". There is no way to do that today without real money.

What exists

• A child in a cell has only its own environment. Its platform (env) credential is empty, and children.modelEgress=openrouter lets it call OpenRouter on the child's own key.
• subscription.budget_usd (#765) caps a sealed key's whole-life spend. A spawn can hand a child a starter key (#747). Both need a real key, which means real money on someone's account.
• model_grant caps an account's monthly spend on one forge's own credential. It does not reach a child.

Wanted

• The parent lends a child its own model credential without the child ever holding it. The parent serves a model endpoint on the beat door or parentLocal, and the child's crew reaches it through a subscription row (api_base = the parent, a per-child bearer minted at spawn, sealed in parent.json like the proposal token).
• The parent meters every call by list price against a $ cap per child, set by an admin on the child's row (/forges, terminal, PUT /api/v1/forges/:name/credit {"usd":200}). At the cap, calls are refused with one sentence: "scottishbob's $200 from its parent is spent; the parent's admin can raise it."
• Both forges show it: the parent's forge list shows spent / cap per child, and the child's crew page shows "funded by its parent: $X of $200 left".

Done when

• A test boots a child cell with a $1 cap. Its crew run reaches the model through the parent with no credential in the child's environment or data directory, and a second run past the cap is refused with the sentence above.
• The cap and the spend read back on both forges' APIs.
• children.modelEgress stays as it is. This adds a path through the parent and opens no new egress.

First use: scottishbob, $200.

Part of P4 #590.

Parked by the crew: the check stayed red through 2 automatic tries (pull #10488 closed; its branch mason/10484-a-parent-funds-a-child-forge-s-crew-a-ca at ace280e228d0 keeps the work so far). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.