Default-enable AI Planner in Host Agent; propose cloud-planner-proxy

- Host Agent now defaults AI_PLANNER_ENABLED=true (opt-out via env),
  scoped to apps/device-host-agent/host_agent/execution.py only; the
  shared runtime.planner_config default (disabled) is unchanged.
- Add openspec proposal for cloud-planner-proxy: centralize LLM
  provider config/credentials on the Cloud Control Plane and let the
  Host Agent proxy AI Planner decisions through it instead of holding
  provider API keys locally. Proposal only, no implementation yet.

openspec/changes/cloud-planner-proxy/specs/agent-runtime/spec.md

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+## MODIFIED Requirements
+
+### Requirement: Pluggable dual-provider tool-calling abstraction
+The system SHALL support at least two interchangeable LLM providers
+(Anthropic native tool use and OpenAI function calling) for the AI
+Planner's decision calls, selectable via configuration, with both providers
+constrained to return exactly one tool call per request. Independently of
+provider selection, the system SHALL support at least two transports for
+making that decision call -- direct-to-provider (the tool-calling client
+calls the provider's SDK itself, using locally configured credentials) and
+cloud-proxy (the tool-calling client calls the Cloud Control Plane's
+planner-decision endpoint, which calls the provider using cloud-held
+credentials) -- selectable via configuration without requiring any change
+to `AIPlanner`'s own decision logic.
+
+#### Scenario: Provider selected via configuration
+- **WHEN** the AI Planner is configured with a given provider identifier
+- **THEN** it constructs and uses the tool-calling client for that provider
+  without requiring any change to `AIPlanner`'s own decision logic
+
+#### Scenario: Provider response resolves to a single decision
+- **WHEN** either supported provider returns a response to a tool-calling
+  request
+- **THEN** the response is parsed into exactly one tool name and one
+  arguments object, regardless of which provider produced it
+
+#### Scenario: Transport selected via configuration
+- **WHEN** the Host Agent is configured with a given transport (direct or
+  cloud-proxy)
+- **THEN** `AIPlanner` is constructed with the tool-calling client for that
+  transport, and its own decision logic is unchanged regardless of which
+  transport is in effect
+
+#### Scenario: Direct transport remains available and default
+- **WHEN** no transport is explicitly configured
+- **THEN** the AI Planner uses the direct-to-provider transport, matching
+  its behavior before the cloud-proxy transport existed
+
+#### Scenario: Cloud-proxy transport resolves a decision without a local provider client
+- **WHEN** the Host Agent is configured with the cloud-proxy transport
+- **THEN** its tool-calling client sends the decision request to the Cloud
+  Control Plane's planner-decision endpoint instead of constructing a local
+  Anthropic or OpenAI SDK client

openspec/changes/cloud-planner-proxy/specs/cloud-planner-proxy/spec.md

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+## ADDED Requirements
+
+### Requirement: Cloud Control Plane exposes an authenticated planner-decision endpoint
+The Cloud Control Plane SHALL expose an internal endpoint that accepts an AI Planner tool-calling decision request from an authenticated Host Agent, and SHALL require the same host-scoped bearer credential already used for heartbeat, claim, lease renewal, and result reporting -- no separate credential or enrollment step.
+
+#### Scenario: Authenticated host requests a planner decision
+- **WHEN** a Host Agent presents its existing valid host-scoped credentials
+  with a planner-decision request
+- **THEN** the Cloud Control Plane accepts and processes the request for
+  that host
+
+#### Scenario: Unauthenticated or foreign-host request is rejected
+- **WHEN** a request omits valid host-scoped credentials, or presents
+  credentials bound to a different host than the one referenced in the
+  request
+- **THEN** the Cloud Control Plane rejects the request without invoking any
+  LLM provider
+
+### Requirement: Endpoint resolves exactly one tool-call decision using cloud-held provider configuration
+The Cloud Control Plane SHALL use its own configured LLM provider, model, and credentials -- not any value supplied by the requesting Host Agent -- to resolve a planner-decision request to exactly one tool name and one arguments object, within the request's timeout.
+
+#### Scenario: Provider returns a usable decision
+- **WHEN** the configured provider responds to a planner-decision request
+  with a tool call
+- **THEN** the Cloud Control Plane returns exactly one resolved tool name
+  and arguments object to the requesting Host Agent
+
+#### Scenario: Configured provider is unreachable or misconfigured
+- **WHEN** the Cloud Control Plane's configured provider call fails (for
+  example, invalid credentials, provider error, or timeout)
+- **THEN** the endpoint returns a structured failure response rather than a
+  fabricated decision, and does not crash the Cloud Control Plane process
+
+### Requirement: Cloud-proxy transport removes the need for Host Agent-held provider credentials
+A Host Agent using the cloud-proxy transport for its AI Planner SHALL be able to execute AI-planned tasks without any locally configured LLM provider API key.
+
+#### Scenario: Host configured for cloud-proxy transport has no local provider key
+- **WHEN** a Host Agent is configured to use the cloud-proxy transport and
+  has no `ANTHROPIC_API_KEY`/`OPENAI_API_KEY` set in its own environment
+- **THEN** it can still obtain AI Planner decisions by calling the Cloud
+  Control Plane's planner-decision endpoint
+
+### Requirement: Planner-decision requests are not durably persisted
+The Cloud Control Plane SHALL process planner-decision requests, including any screenshot and prompt text they carry, without durably persisting that screenshot or prompt content; only request metadata (such as host identifier, resolved tool name, latency, and error classification) may be retained for observability.
+
+#### Scenario: Request handling completes without storing prompt or screenshot content
+- **WHEN** the Cloud Control Plane finishes handling a planner-decision
+  request
+- **THEN** the raw prompt text and screenshot bytes from that request are
+  not present in any durable store or log the Cloud Control Plane retains
+
+### Requirement: Planner-proxy failures do not silently substitute a default action
+The Cloud Control Plane SHALL report a failure to the requesting Host Agent when it cannot resolve a planner-decision request to a valid tool call, rather than returning a default, guessed, or previously cached decision.
+
+#### Scenario: Endpoint cannot resolve a decision
+- **WHEN** the configured provider does not return a usable tool call for a
+  planner-decision request
+- **THEN** the Cloud Control Plane's response indicates failure, and the
+  requesting Host Agent treats the planner call for that turn as failed