feat(core): add shared interaction transports

[drewstone]

What changed

• add one public interaction broker for permissions and questions
• add loopback HTTP and optional MCP transports
• bind requests to canonical Agent Interface identities and digests
• enforce timeout, teardown, malformed-input, authentication, and body-size behavior
• keep the MCP SDK optional for ordinary Agent Core consumers

This moves reusable behavior out of the Sandbox monorepo. Sandbox adapters can now keep only runner-specific wiring and durable plan storage.

Proof

• Agent Core: 22 test files, 503 tests passed
• TypeScript source and test projects passed
• build passed
• packed tarball contains both interaction entry points and declarations
• clean consumer proof did not install the optional MCP SDK
• merge-tree against current main passed

[tangletools]

✅ Auto-approved drewstone PR — 2b536883

This PR was opened by the trusted drewstone account.
The full PR reviewer audit still runs separately and will publish findings if it detects issues.

This approval is provisional. It rests on the audit running. If the audit cannot run — for example the CLI bridge rejects it — this approval is dismissed rather than left standing, so an unrun check never reads as a passing one.

_{tangletools · auto-approval · reason: drewstone_author · 2026-08-15T10:11:47Z}