refactor: retire the external host-adapter contract and Hermes #324

Why

v5 replaces adapters with first-class hosts (design Decision 10). One card: the CLI, admission, execution and tests share one contract.

In scope

Remove ak host adapters, ak x aqe-provider, AK_EXPERIMENTAL_HOST_ADAPTERS, adapter routing in ak run, the ten adapter modules, the external hook-audit provider and external lifecycle entries, about 15 test files and three guides; mark ADR-0029 and ADR-0031 withdrawn from v4.

Out of scope

Built-in host registry, bindings and lifecycle.

Acceptance criteria

• The listed modules and guides are gone and the built-in hosts still pass
• ADR-0029 and ADR-0031 say "Withdrawn from v4; carried to v5"
• pnpm test passes

Dependencies

Blocked by: #323

Blocks: #330, #385

Plan

• Release: 4.0.0-beta.1
• Size: size:L · Workstream: ws:project-scope
• Epic: feat: project-scope P0, removals and the enforcement baseline (epic) #295
• Card: P0-04 in the card inventory
• Sources: Design (docs(plans): project-scoped management design and its prerequisites #289) Setting host adapters aside for v5; Supersession ledger (docs(plans): v4 GA master plan, supersession ledger and v5 registers #290) G1

Verification

• pnpm test