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

@@ -0,0 +1,43 @@
+## 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