feat(runtime): add AgentLocality and ScheduledBackend for distributed scheduling

[chaizhenhua]

Summary

• Add AgentLocality enum to AgentSpec with Local, Remote, and Distributed variants
• Add ScheduledBackend implementing AgentBackend for Worker-dispatched agent execution
• Extend resolve pipeline to select backend based on agent locality

Motivation

Enable agents to be scheduled to distributed Worker Nodes (e.g., Claude Code, Codex CLI) rather than executing only in-process or on fixed remote endpoints. This is the framework-level support for the distributed CLI agent architecture.

Changes

• crates/awaken-contract/src/registry_spec.rs — add AgentLocality, DistributedConfig, SchedulingPolicy types (+222 lines)
• crates/awaken-runtime/src/extensions/a2a/scheduled_backend.rs — new ScheduledBackend impl (+399 lines)
• crates/awaken-runtime/src/registry/resolve/pipeline.rs — route Distributed locality to ScheduledBackend (+73/-28 lines)
• crates/awaken-runtime/src/extensions/a2a/mod.rs — module export (+2 lines)
• crates/awaken-runtime/src/registry/resolver.rs — add locality field default (+1 line)

Test plan

• Unit tests for AgentLocality serialization roundtrip
• ScheduledBackend integration with mock ScheduledBackendFactory
• Resolve pipeline selects correct backend for each locality variant