docs(skills): platform feature-flags example teaches a non-existent authoring surface

[os-zhuang]

Context

Surfaced by the new check:skill-examples gate (#3094 / #3215's sibling PR #3224). When tagging self-contained skill examples for type-checking, the feature-flags example in skills/objectstack-platform/SKILL.md failed to compile against @objectstack/spec. Unlike the other drifts fixed in that PR (simple wrong import paths), this one is a deeper, structural problem, so the block was deliberately left as-authored and untagged rather than shipping a guessed correction. This issue tracks the real fix.

The example (paraphrased)

import { defineStack } from '@objectstack/spec';

export default defineStack({
  featureFlags: [
    {
      name: 'experimental_ai_copilot',
      enabled: true,
      strategy: 'percentage',
      conditions: { percentage: 25 },
      environment: ['production'],
    },
    // ...
  ],
});

Why it's wrong (three concrete problems)

1. featureFlags is not a key on the defineStack input. ObjectStackDefinitionInputSchema (in packages/spec/src/stack.zod.ts) has no featureFlags (nor features) top-level key. A consumer copying this hits TS2353: 'featureFlags' does not exist in type 'ObjectStackDefinitionInput'.

2. The only related schema is not an authoring input. features: z.array(FeatureFlagSchema) lives inside KernelCapabilitiesSchema → mounted at ObjectStackCapabilitiesSchema.system (packages/spec/src/stack.zod.ts). But ObjectStackCapabilitiesSchema is a runtime capabilities descriptor — it's exported only as a type and is not referenced by ObjectStackDefinitionInputSchema. So feature flags currently have no authoring home in objectstack.config.ts via this path. Renaming featureFlags → features does not fix it (verified: features is not a top-level input key either).

3. environment shape/value is also wrong. FeatureFlagSchema.environment (packages/spec/src/kernel/feature.zod.ts) is a single enum z.enum(['dev','staging','prod','all']).default('all') — not an array, and 'production' is not a valid value ('prod' is). The example writes environment: ['production'].

What a fix needs to decide (not a mechanical rename)

The core question is a product/spec decision: how (or whether) feature flags are authored in a stack. Options include:

• Add a real top-level (or nested) authoring key to ObjectStackDefinitionInputSchema that accepts FeatureFlagSchema[], then rewrite the example against it; or
• If feature flags are not author-declared (only a runtime descriptor), remove/replace the example with the actual mechanism (e.g. how flags are seeded/registered at runtime).

Whoever owns the feature-flags surface should confirm the intended authoring path before the example is rewritten.

Definition of done

• The skills/objectstack-platform/SKILL.md feature-flags example compiles against @objectstack/spec (correct key, correct environment scalar, valid enum value), or is replaced with the real mechanism.
• Re-add the <!-- os:check --> marker above the block so check:skill-examples covers it going forward.

Refs: PR #3224 (the gate + the deferral note in its description).