feat(core): ContentPart + parts() helper for multi-format result envelopes

[vreshch]

Summary

Formalizes the convention surfaced during the admin-agents UX walk: the final `result` event can carry multiple MIME-typed views of the same output. Order = render preference.

```ts
import { result, parts } from '@agentage/core';

yield result(true, parts({
markdown: '# Done\n\n42 commits',
json: { commits: 42 },
}));
```

Backward compatible — `result(success, output?: unknown)` still accepts any value.

API additions

• `ContentPart` type: `{ type: string; content: unknown }`
• `ResultOutput` alias: `unknown` (with docs explaining the array convention)
• `parts({ markdown, text, json, extra })` factory — builds a `ContentPart[]` in canonical preference order

Canonical MIME types: `text/markdown` → `text/plain` → `application/json`. Non-canonical parts pass through `extra`.

Bumps `@agentage/core` to 0.10.0 (minor — additive only).

Cross-repo follow-ups

• `agentage/web#235` (in flight) — `ResultEvent` detects array-of-parts, renders preferred view with JSON toggle.
• `agentage/agents#5` (in flight) — 5 TS agents emit `result(true, parts({...}))`.

Test plan

• `npm run verify` green (151 tests pass, including 6 new tests for `parts()` + ContentPart)
• Existing `result()` callers in CLI / hub / e2e remain green (no signature change)

[github-actions]

🎉 PR Validation ✅ PASSED

Commit: 7c86257f897bf091d5be672a1756ef5b10cb0ce6
Branch: feature/result-content-parts

Checks:

• ✅ Release guard (no version/changelog changes)
• ✅ Dependencies installed
• ✅ Type check passed
• ✅ Linting passed
• ✅ Format check passed
• ✅ Tests + coverage passed
• ✅ Build successful

Ready to merge! ✨

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⏰ Generated at: 2026-04-26T18:06:51.620Z