feat(cli): read stack outputs in one call and derive repo show's default geography

### Component

Scripts / CLI (`cli/src/stack-outputs.ts`, `cli/src/platform-doctor.ts`, `cli/src/repo-display.ts`)

### Describe the feature

Two related inefficiencies in how the CLI reads stack outputs.

**1. One `DescribeStacks` call per output.** `getStackOutput` issues a fresh `DescribeStacksCommand` each time and caches nothing:

```ts
// cli/src/stack-outputs.ts
const result = await cf.send(new DescribeStacksCommand({ StackName: stackName }));
```

`runPlatformDoctor` now reads nine outputs, so one `bgagent platform doctor` makes nine calls that each return the same full stack description. They run concurrently in a `Promise.all`, so the wall-clock cost is small, but it is nine API calls where one would do — and `DescribeStacks` is a throttled control-plane operation, so it is the kind of thing that degrades on a busy account rather than on a developer laptop.

**2. `repo show` reports a hardcoded default geography.** `PLATFORM_REPO_DEFAULTS.model_id` is a compile-time literal:

```ts
// cli/src/repo-display.ts
model_id: 'global.anthropic.claude-opus-5',
```

The effective default is `.`, so on a deployment using any other geography `repo show` displays a model the stack does not grant for every repo that sets no override. Correct on the shipped default only.

### Use case

The second is the user-visible one: an operator on a non-`global` deployment reads `repo show`, sees `global.anthropic.claude-opus-5`, and has been told the wrong thing about their own stack — the same class of "documented value diverges from deployed value" the geography work otherwise closes. The first is cheap hygiene that gets slightly worse each time a check is added.

### Proposed solution

- Fetch the stack once and read outputs from that result (either a `describeStack`-level helper that returns all outputs, or a short-lived per-invocation cache keyed by region+stack). `getStackOutput` already has a `getStack` internal that would be the natural seam.
- Derive the displayed default from the stack's `BedrockGeoRegion` output when available, and fall back to the literal only when it is absent (the same degradation `repo onboard --model` already uses for older stacks).

### Other information

Both raised during review of #806 and deliberately deferred: the PR's scope is making an unusable model fail early rather than at turn 0, and neither of these changes that behaviour. The `BedrockGeoRegion` / `BedrockModelIds` outputs that #806 adds are what make the second one implementable.

Inizia da cli/src/stack-outputs.ts e dal suo getStack interno esistente, quindi segui i chiamanti in cli/src/platform-doctor.ts e cli/src/repo-display.ts. Verifica come vengono esposti BedrockGeoRegion e gli altri output dello stack, e analizza il fallback esistente di repo onboard. Il lavoro è completato quando una descrizione dello stack serve per le letture di platform doctor e repo show ricava la propria geografia predefinita dallo stack, ricorrendo al fallback quando l’output è assente.

Scritto dal modello di indicizzazione a partire dal testo della issue.