refactor(tools): move model-interaction tools to the agent_tools registry (#4445)

Moves chat_with_model, ask_teacher and list_models out of ai_interaction.py
into src/agent_tools/model_interaction_tools.py (the do_ prefix dropped) and
registers them in TOOL_HANDLERS, so dispatch flows through the registry instead
of the dispatch_ai_tool elif in tool_execution.py.

The implementations are relocated, not wrapped. ai_interaction.py keeps only
the shared helpers they reuse (_resolve_model, AI_CHAT_TIMEOUT), still used by
the not-yet-migrated session/pipeline tools. dispatch_ai_tool loses its three
now-unused branches.

Also removes the dead do_second_opinion: it was already off the live tool
surface (no tag/schema/parsing/dispatch; tool_index.py notes it was removed),
so the function and its stale frontend catalog entries (admin.js, assistant.js)
are deleted.

Tests: owner-scope test points at the new list_models location and drops the
moved tools from the dispatch_ai_tool parametrize; a new
test_model_interaction_registry covers registration, owner threading, and
registry dispatch.

static/js/admin.js

@@ -1756,7 +1756,6 @@ const TOOL_META = {
   manage_skills:     { name: 'Skills',           desc: 'Learn and use procedures',        cat: 'Knowledge',  ctx: '~200' },
   manage_rag:        { name: 'RAG / Docs',       desc: 'Query indexed documents',         cat: 'Knowledge',  ctx: '~150' },
   chat_with_model:   { name: 'Chat with Model',  desc: 'Talk to another AI model',        cat: 'Multi-Agent', ctx: '~200' },
-  second_opinion:    { name: 'Second Opinion',   desc: 'Get another model\'s take',       cat: 'Multi-Agent', ctx: '~150' },
   pipeline:          { name: 'Pipeline',         desc: 'Multi-step AI workflows',         cat: 'Multi-Agent', ctx: '~200' },
   ask_teacher:       { name: 'Ask Teacher',      desc: 'Query a more capable model',      cat: 'Multi-Agent', ctx: '~150' },
   send_to_session:   { name: 'Send to Session',  desc: 'Send message to another chat',    cat: 'Sessions',   ctx: '~100' },