docs: update tool-adding instructions for auto-discovery

- AGENTS.md: 3 files → 2 files, remove _discover_tools() step
- adding-tools.md: remove Step 3, note auto-discovery
- architecture.md: update discovery description
- tools-runtime.md: replace manual list with discover_builtin_tools() docs
- hermes-agent skill: remove manual import step

AGENTS.md

@@ -13,7 +13,7 @@ source venv/bin/activate  # ALWAYS activate before running Python
 ```
 hermes-agent/
 ├── run_agent.py          # AIAgent class — core conversation loop
-├── model_tools.py        # Tool orchestration, _discover_tools(), handle_function_call()
+├── model_tools.py        # Tool orchestration, discover_builtin_tools(), handle_function_call()
 ├── toolsets.py           # Toolset definitions, _HERMES_CORE_TOOLS list
 ├── cli.py                # HermesCLI class — interactive CLI orchestrator
 ├── hermes_state.py       # SessionDB — SQLite session store (FTS5 search)
@@ -181,7 +181,7 @@ if canonical == "mycommand":
 
 ## Adding New Tools
 
-Requires changes in **3 files**:
+Requires changes in **2 files**:
 
 **1. Create `tools/your_tool.py`:**
 ```python
@@ -204,9 +204,9 @@ registry.register(
 )
 ```
 
-**2. Add import** in `model_tools.py` `_discover_tools()` list.
+**2. Add to `toolsets.py`** — either `_HERMES_CORE_TOOLS` (all platforms) or a new toolset.
 
-**3. Add to `toolsets.py`** — either `_HERMES_CORE_TOOLS` (all platforms) or a new toolset.
+Auto-discovery: any `tools/*.py` file with a top-level `registry.register()` call is imported automatically — no manual import list to maintain.
 
 The registry handles schema collection, dispatch, availability checking, and error wrapping. All handlers MUST return a JSON string.