feat(agent-test): define version 1 contracts and profiles

Description

The pitch.

## Problem

External-agent tests need a strict, versioned contract whose wire bytes, profile identity, result semantics, and extension boundary are reviewable independently of transport and CLI code.

## Change

- define the version 1 driver, observation, profile, and result models
- add codec-bound validation, deterministic schemas, golden fixtures, and the packaged capability profile
- bind results and evidence to the exact profile digest
- keep package release versions unchanged; these are contract generation identifiers

## Verification

- final stack: `make ci-local` — all 5 checks passed; 2,081 passed, 2 skipped, 1 deselected
- deterministic schema generation/check and profile digest recomputation pass
- independent contract/schema review found no Critical or Important issues

## Stack

**2 of 9. Depends on #220.** PR 3 targets this branch. This PR defines contracts only; it does not add network transport.

**Merge note:** After #220 merges into `main`, retarget this PR to `main`, wait for its CI, and only then merge it.