feat: per-role tool lists and system prompt overlays

Each role in model_registry.json can now carry two optional keys:
  system_append — injected into the system prompt at position 7 (after
                  memory, closest to the turn) for the active chat_role
  tools         — explicit tool allow-list; intersected with the user's
                  access-level filter so it can only restrict, never elevate

No changes needed for existing users — missing keys fall back to current
behavior. Add keys to a role to give it a specialty focus:

  "coder": {
    "primary": "claude_cli",
    "system_append": "You are in code-specialist mode...",
    "tools": ["web_search", "file_read", "shell_exec", "scratch_write"]
  }

Changes:
- model_registry.py: get_role_config() returns system_append + tools
- context_loader.py: role_append param appended as "--- Role Context ---"
- tools/__init__.py: get_tools_for_role/get_openai_tools_for_role accept
  optional tool_list and intersect with access-level filter
- orchestrator_engine.py: tool_list threaded through run/resume/checkpoint
- openai_orchestrator.py: tool_list threaded through run/resume/checkpoint;
  _build_client now calls get_openai_tools_for_role instead of returning
  unfiltered OPENAI_TOOL_SCHEMAS
- routers/orchestrator.py: pulls role_cfg for chat_role, passes both
  role_append and tool_list to context loader and engine

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cortex/context_loader.py

@@ -17,6 +17,7 @@ def load_context(
     include_long: bool = True,
     include_mid: bool = True,
     include_short: bool = True,
+    role_append: str = "",
 ) -> str:
     """
     Build the system-prompt context block for a given tier and memory toggles.
@@ -28,6 +29,9 @@ def load_context(
     Tier 2  — + USER full + PROTOCOLS + memory      (~5,000 tokens)
     Tier 3  — + last 2 raw session logs             (~15,000 tokens)
     Tier 4  — + last 7 raw session logs             (~50,000 tokens)
+
+    role_append — optional text injected last (closest to the turn),
+                  sourced from the active role's system_append config.
     """
     inara_dir = persona_path()
     parts = []
@@ -107,4 +111,8 @@ def load_context(
             for sf in session_files:
                 parts.append(f"--- Session: {sf.name} ---\n{sf.read_text()}")
 
+    # ── 7. Role-specific instructions (always last — closest to the turn) ──
+    if role_append and role_append.strip():
+        parts.append(f"--- Role Context ---\n{role_append.strip()}")
+
     return "\n\n".join(parts)