feat(actor): extend hierarchical actor YAML schema and loader

Extend actor YAML schema to support hierarchical graphs with explicit
node types (agent, tool, conditional, subgraph), per-node LSP bindings
(lsp_binding with server, languages, auto, capabilities), and tool-source
references (skills, mcp_servers, agent_skills).

Add schema validation for namespaced actor references, duplicate node IDs,
edge target existence, and graph reachability — all nodes must be reachable
from entry_node via explicit edges or conditional node routing targets.

Update loader to report YAML parse errors with precise line/column positions
and schema validation errors with dotted field paths and remediation hints
pointing to docs/reference/actor_config.md.

Add docs/reference/actor_config.md as the practical configuration reference
covering hierarchical graph examples, node type table, topology rules, and
common error cases with fix guidance.

Refresh examples/actors/graph_workflow.yaml to replace deprecated actor_path
with actor_ref. Add benchmarks/actor_yaml_bench.py for schema load overhead.

Tests: 95 Behave scenarios, 10 Robot smoke tests (including hierarchical
loader smoke test), security scan clean, coverage 99% (threshold 97%).

ISSUES CLOSED: #157

examples/actors/graph_workflow.yaml

@@ -134,8 +134,7 @@ route:
       type: subgraph
       name: Code Reviewer
       description: Runs code review workflow
-      config:
-        actor_path: examples/actors/simple_llm.yaml
+      actor_ref: local/code-reviewer
 
     # Escalate if tests keep failing
     - id: escalate

examples/actors/hierarchical_workflow.yaml

@@ -0,0 +1,102 @@
+# Hierarchical Actor - Multi-Agent Development Workflow
+# Demonstrates LSP bindings, tool-source references, subgraph nodes,
+# and skills integration in a graph actor.
+
+name: workflows/dev-workflow
+type: graph
+description: >-
+  Multi-agent development workflow with a strategist, implementer, and
+  reviewer. Each node has fine-grained LSP and tool-source control.
+version: "1.0"
+model: gpt-4
+
+# Actor-level skill references
+skills:
+  - local/file-ops
+  - local/git-ops
+
+# Actor-level LSP binding (default for all nodes)
+lsp:
+  - local/pyright
+lsp_capabilities:
+  - diagnostics
+  - hover
+  - definitions
+  - references
+lsp_context_enrichment:
+  diagnostics: true
+  type_annotations: false
+  max_diagnostics_per_file: 50
+
+route:
+  nodes:
+    - id: strategist
+      type: agent
+      name: Strategy Planner
+      description: Plans the implementation strategy using read-only LSP
+      config:
+        model: gpt-4
+        prompt: |
+          Plan the implementation strategy. Use LSP diagnostics to assess
+          codebase health before proposing changes.
+      lsp_binding:
+        server: local/pyright
+        languages:
+          - python
+        capabilities:
+          - diagnostics
+          - hover
+      tool_sources:
+        - type: skill
+          name: local/file-ops
+
+    - id: implementer
+      type: agent
+      name: Code Implementer
+      description: Implements code changes with full LSP and tools
+      config:
+        model: gpt-4
+        prompt: |
+          Implement the changes as planned. Use full LSP capabilities
+          for navigation and refactoring.
+      lsp_binding:
+        auto: true
+      tool_sources:
+        - type: skill
+          name: local/file-ops
+        - type: skill
+          name: local/git-ops
+        - type: mcp
+          name: local/filesystem
+
+    - id: reviewer
+      type: subgraph
+      name: Code Review
+      description: Delegates to the code-reviewer actor for review
+      actor_ref: local/code-reviewer
+
+  edges:
+    - from_node: strategist
+      to_node: implementer
+    - from_node: implementer
+      to_node: reviewer
+
+  entry_node: strategist
+  exit_nodes:
+    - reviewer
+
+context_view: strategist
+memory:
+  enabled: true
+  max_messages: 50
+  max_tokens: 16000
+  summarize_old: true
+
+context:
+  include_dirs:
+    - src/
+    - tests/
+  exclude_patterns:
+    - "**/__pycache__/**"
+    - "*.pyc"
+  max_context_tokens: 32000