Browser UI: the Products tab's "every decision about it" link opens an empty custody chain

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`engine/ui/src/governor/ProductsScreen.tsx` links each product's custody as `CustodyLink subject={`product:${name}`}`. The engine does not resolve that subject. `rocky audit --for product:revenue_daily` on the fulfillment POC answers `subject_kind: "model"`, `resolved: false`, 0 decisions, while `draft:revenue_daily` resolves to 5.

Why: `commands/audit.rs` classifies a subject as a plan when a ledger row's `plan_id` equals it (or a run id, or a plan file on disk), else as a model. The product loop records its decisions with `plan_id = "draft:"` (rocky-mcp `tools.rs`); `product:` is the row's `product_id`, which custody never consults. So the link the tab offers as "every decision about it" opens a chain with nothing in it, and the page cannot tell that from a product nobody decided on.

Two fixes, not exclusive:

1. **UI, now.** Link to what the engine already answers for a product: the audit tab filtered by product (`GET /api/v1/audit?product=` matches `rocky audit --product `), which is exactly "every decision about it". Keep a custody link only for a subject custody resolves.
2. **Engine, later.** Teach custody a product subject: `product:` resolves through `product_id`, with `subject_kind: "product"` in `AuditOutput` (that is a schema change: `just codegen` + fixtures). Then the tab can link custody again.

Found by the docs lane while writing `guides/browser-ui.md` (#2000); the guide's custody example now uses a subject that resolves.

Start in engine/ui/src/governor/ProductsScreen.tsx and compare the product link with GET /api/v1/audit?product=. Read commands/audit.rs and rocky-mcp tools.rs to understand the existing subject and plan identifiers; if pursuing the engine option, also inspect AuditOutput, codegen, and fixtures. Done means the Products tab no longer opens an empty custody chain for a product, or product custody resolves consistently if that option is chosen.

Written by the indexing model from the issue text.