Add RuntimeConfig three-toggle switch layer

[H-maximedelpit]

Fork of PR #185 (local mode / Client.local) — opened as a Draft so it does not touch that PR's own branch or implementation.

Adds hai_agents_runtime: a small, hand-written sibling package to pick where each of the three parts of an agent task runs — agent, environment, model — and get an ordinary Client back. Instantiate RuntimeConfig; presets cloud()/local() are shorthand; override any toggle with .with_().

• Inference toggle is deliberately simple: H's cloud (Inference.cloud()) or a model you host by URL (Inference.self_hosted(url)).
• Cloud agent -> Client() (unchanged). User-device agent -> Client.local() (from [claude] Local mode: Client.local() + LocalRuntime (generated from sdk-codegen) #185), with a self-hosted model endpoint handed to the runtime at spawn and the cloud key withheld.
• The local agent runtime ships as an opt-in extra: local-agent (compiled binary, public) / local-agent-src (source, internal via private CodeArtifact). A user_device config fails at construction with a clear message if neither is installed.
• Crossing rule kept: a config that would send a screenshot across the internet twice per step is refused at construction.

Builds on #185 (Client.local); the runtime spawn_env hook is hcompai/agent_platform#1452. Regen-proof hand-written sibling, not inside hai_agents. Draft — experimental, interface may change.

Depends on / builds on

• Local mode Client.local() + LocalRuntime → [claude] Local mode: Client.local() + LocalRuntime (generated from sdk-codegen) #185
• SDK local-mode codegen + spawn_env → hcompai/agent_platform#1452
• Runtime consumer-contract mirror → hcompai/agent_platform#1451
• Runtime consumer-contract gate → hcompai/agent_platform#1439
• Loopback-http webhooks for local runtimes → hcompai/agent_platform#1449

[adeprezh]

lgtm, will need a bump of agent-interface and agp-types before merging:

uv lock -P agent-interface
uv lock -P agp-types
uv sync

[abonneth]

Superseded by #222