scaffold+broker: dedicated <ns>.balance method for managed budget apps

[Alexgodoroja]

Problem

Managed (credit-metered) apps meter a per-user $-budget and surface it on the X-Pilot-Credits-Remaining header — but a keyless adapter only returns the response body, so an agent can't actually read that header. There was no first-class way to answer "how much budget do I have left?" before a spend op.

The broker already had half of this: serveCreditBalance + an optional credit.balance_path to shadow a partner's account-balance endpoint (so it returns per-user budget instead of leaking the pooled account). But it was opt-in with no default and no adapter-facing method — dormant.

This PR — wires it end to end

• Broker: answers a canonical /_pilot/balance for any credit app, from its own per-caller ledger — no upstream call, no master key, no debit, never a 402, scoped to the caller (pooled balance never disclosed). Reuses the existing serveCreditBalance; the canonical path is always available alongside the optional creditBalancePath shadow.
• Scaffolder: auto-injects <ns>.balance into every auth: managed app (no submission field) — a GET to /_pilot/balance that flows through registration, the manifest exposes list, and <ns>.help like any authored method.
• scaffold.BalanceMetaPath == broker.pilotBalancePath keeps both ends in sync.

Response body:

{ "balance": "$1.80", "credits_remaining": 1800000, "credits_seed": 5000000,
  "unit": "micro_usd", "scope": "per-pilot-user" }

Tests

• Broker (zz_balance_meta_test.go): canonical path seeds a fresh caller, reflects a spend, is free, and never forwards upstream.
• Scaffold (zz_balance_method_test.go): injected for managed apps only (not byo), generated + wired to the reserved path + in the manifest, and the project compiles.
• Full broker + scaffold + publish suites green.

Docs

MANAGED-KEY.md and PUBLISHING-PLAYBOOK.md document the pattern for all broker apps that meter a budget: set the credit block → the adapter exposes <ns>.balance for free; tell agents to check it (or the header) before a spend.