Bug Description

When a plugin registers a tool into an existing built-in toolset, the tool can exist in the live registry but still be missing from session tool definitions.

Observed case:

• plugin: image_advanced
• tool: image_generate_advanced
• target toolset: image_gen

The plugin was enabled and loaded successfully, and the tool was present in the live registry under image_gen, but new sessions exposed only image_generate and not image_generate_advanced.

Steps to Reproduce

1. Enable the bundled standalone plugin image_advanced.
2. Start a fresh Hermes session with image_gen enabled.
3. Verify these three layers:
   • registry.get_tool_names_for_toolset('image_gen')
   • toolsets.resolve_toolset('image_gen')
   • model_tools.get_tool_definitions(enabled_toolsets=['image_gen'])
4. Observe that the registry layer includes image_generate_advanced, while resolved toolsets and final tool definitions do not.

Expected Behavior

If a plugin registers a tool into an existing built-in toolset, that tool should be included in:

• get_toolset(...)
• resolve_toolset(...)
• final session tool definitions

Actual Behavior

Built-in toolsets are returned directly from static TOOLSETS, so plugin-added tools for that same toolset are dropped during resolution.

Example from a real run:

• registry.get_tool_names_for_toolset('image_gen') -> ['image_generate', 'image_generate_advanced']
• resolve_toolset('image_gen') -> ['image_generate']
• get_tool_definitions(enabled_toolsets=['image_gen']) -> only image_generate

Root Cause

toolsets.get_toolset(name) returns the static built-in definition immediately when name in TOOLSETS.
That means it never merges in live registry membership for plugin-added tools targeting the same toolset.

So the system correctly loads the plugin and registers the tool, but the toolset resolver silently discards it.

Minimal Fix

For built-in toolsets, merge:

• static TOOLSETS[name]['tools']
• live registry.get_tool_names_for_toolset(name)

before returning the toolset definition.

Notes

I verified a local patch that makes get_toolset('image_gen'), resolve_toolset('image_gen'), and get_tool_definitions(enabled_toolsets=['image_gen']) all include both tools.

I also added a regression test covering the case where a plugin adds a tool to an existing built-in toolset.