feat: Add makeDiscoverableExo constructor

[grypez]

Closes: #649

This PR adds the makeDiscoverableExo constructor, which associates to a capability interface the sort of descriptive metadata needed by @ocap/kernel-agents to perform agentic capability composition.

---

Note

Adds makeDiscoverableExo with method schemas and an agent discover utility that converts discoverable exos into capability records, updating schemas/exports, tests, and examples.

• Kernel Utils (@metamask/kernel-utils):
  • Discoverable Exos: Add makeDiscoverableExo and DiscoverableExo in src/discoverable.ts with runtime describe() for method schemas; export via src/index.ts and new exports./discoverable entry.
  • Schema Types: Add src/schema.ts defining JsonSchema (with items) and MethodSchema; re-export types.
  • Tests: Add src/discoverable.test.ts; update src/index.test.ts to include new exports.
• Kernel Agents (@ocap/kernel-agents):
  • Discovery Capability: Add src/capabilities/discover.ts that introspects a DiscoverableExo via E(...).describe() and builds a CapabilityRecord; export discover from src/index.ts.
  • Schema Alignment: Update example capabilities (examples.ts, math.ts) to use items in array schemas.
  • Deps: Add @endo/eventual-send and reference updates.
  • Tests: Update src/index.test.ts to expect discover export.
• Kernel Test:
  • Vat Example: Add src/vats/discoverable-capability-vat.js demonstrating a calculator discoverable exo; update TS project references.
• Tooling: Adjust coverage thresholds in vitest.config.ts.

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[sirtimid]

LGTM!