hermes - 💡(How to fix) Fix Persist session-scoped model/reasoning switches in SessionEntry instead of in-memory dict

Problem

Session-scoped model switches (/model, model_switch tool with scope=session) are stored in an in-memory dict (_session_model_overrides / _session_reasoning_overrides) on the GatewayRunner instance. This means:

1. Gateway restart → session model switch is lost. The session itself survives via sessions.json, but the model override silently reverts to config.yaml defaults.
2. SoT is fragmented. Model state lives in a separate dict from the session it belongs to, requiring manual .pop() at 3+ sites (auto-reset, compression-exhausted, /new).
3. Override layering is opaque. config.yaml default → resolve_runtime_provider → _session_model_overrides → agent instance makes it hard to trace what decided the model.

Proposed Solution

Add 3 nullable fields to SessionEntry:

class SessionEntry:
    # ... existing fields ...
    model: Optional[str] = None             # None → config.yaml default
    provider: Optional[str] = None          # None → config.yaml default
    reasoning_effort: Optional[str] = None  # None → config.yaml default

At agent creation time:

model = session_entry.model or config_default_model
provider = session_entry.provider or config_default_provider
runtime_kwargs = resolve_runtime_provider(provider)   # derives api_key/base_url/api_mode
reasoning = parse_reasoning_effort(session_entry.reasoning_effort) or config_reasoning

What this removes

• _session_model_overrides dict + 3 manual .pop() sites
• _session_reasoning_overrides dict + related management code
• _apply_session_model_override() method
• _is_intentional_model_switch() → session_entry.model == agent_model

What this fixes

• Persistence: sessions.json already serializes SessionEntry → model survives gateway restarts
• Single SoT: model is a session attribute, not a side-channel override
• Lifecycle simplification: /new calls reset_session() which creates a fresh SessionEntry with None fields → automatic cleanup, no manual pop needed

Design notes

• api_key/base_url/api_mode are not stored — they are derived from the provider name at runtime via resolve_runtime_provider(). No credentials in session store.
• The runtime_update_callback closure in run.py would write to session_entry.model/.provider/.reasoning_effort + trigger a session store flush instead of mutating the dict.
• TUI gateway _apply_model_switch follows the same pattern.