refactor(session): simplify execution and state workflows

[kitlangton]

What

Make Session execution and shared state workflows express transport decisions, state creation, pure projections, and no-op guards directly. The refactor preserves durable event ordering, cooperative-concurrency boundaries, State finalization, Form errors, and Session projections.

How

• Replace the double-negative WebSocket eligibility calculation with hasHttpHooks.
• Return and reuse a named ToolState created by startToolInput.
• Express inbox admission's pure projection with Effect.map.
• Inline State's single-use transform wrapper and return the interface directly.
• Implement Form's required lookup with Effect.fromOption.
• Use early guards for missing assistant and retry state.

Scope

Behavior-preserving Session/Core cleanup only; no transport policy, durable events, prompt admission, state lifecycle, form settlement, or projection changes.

Testing

• Session execution tests: 67 passed
• State, Form, and Session projector tests: 28 passed
• Simplify follow-up: bun run test test/session-runner-tool-events.test.ts (17 passed)
• bun typecheck from packages/core
• Push hook: bun turbo typecheck --concurrency=3 (40-package workspace)