feat: add runtime models.json config, SDK crate, and extension/session-tree/agent-context primitives

[jaikoo]

Summary

Implements five phases of missing primitives for building an autonomous AI coding harness comparable to pi-mono/coding-agent.

Phase 1: Runtime models.json Config

• Load custom providers from ~/.claw/models.json and .claw/models.json
• Support custom base URLs, API keys (literal or env var), and model definitions
• Provider-prefixed lookup (e.g. ollama/llama3.1:8b) and bare ID matching
• Merged discovery: user-level + project-level providers coexist; same-key project overrides user
• Custom provider routing in ProviderClient respects the api field (anthropic-messages vs openai-completions)
• Hooks into metadata_for_model, detect_provider_kind, max_tokens_for_model, model_token_limit

Phase 2: SDK Crate (rust/crates/sdk/)

• AgentSession wraps ConversationRuntime with event-driven lifecycle
• EventBus provides multi-subscriber broadcast channels for session events
• SessionManager handles CRUD for persisted sessions
• ToolRegistry and SdkToolExecutor for tool registration/execution stubs

Phase 3: Extension System

• Extension trait with lifecycle hooks (on_turn_start, on_turn_complete, on_error)
• ExtensionRegistry for managing extensions and collecting tools
• SimpleExtension for basic tool registration

Phase 4: Session Tree

• Branching/forking/navigation using single-source-of-truth BTreeMap
• Children stored as ID references (not duplicated node data)
• Fork at any node, navigate between branches

Phase 5: Inter-Agent Communication

• AgentContext (thread-safe KV store via Arc<RwLock>)
• AgentTask with lifecycle (pending -> completed/failed)
• TaskRegistry for managing sub-agent tasks

Test Coverage

• 7 e2e tests for models_file (including merge and override scenarios)
• 27 SDK unit tests
• 4 models_file unit tests
• All 1,072 workspace tests pass

Code Review Findings Fixed

• P1: AgentSession::new now uses the passed tool_registry instead of creating an empty one
• P1: Custom provider routing now respects the api field (routes anthropic-messages to AnthropicClient)
• P1: discover_and_load_models now merges user-level and project-level providers instead of replacing
• P1: SessionTree stores children as ID references to avoid data duplication desync