fix(kernel): resolve "default" provider in fallback_models before driver init

[Reaster0]

Summary

• Fallback model entries with provider = "default" were passed verbatim to create_driver(), which only recognises real provider names (ollama, openai, anthropic, …)
• The primary model already resolves "default" → kernel config at spawn_agent(), but the fallback loop was missing the same resolution — causing every bundled agent with a "default" fallback to silently lose all fallback drivers:

  WARN Fallback driver 'default' failed to init: Unknown provider 'default'
  

• This is particularly visible for users whose config.toml sets a non-standard default provider (e.g. ollama pointing at a local proxy), where the fallback is the only recovery path

Changes

• Mirror the primary-model overlay logic for fallback entries: resolve provider, model, api_key_env, and base_url from config.default_model when the fallback specifies "default" or empty string
• Inherit base_url from default_model before falling back to lookup_provider_url(), so custom endpoints propagate correctly
• Use resolved values in strip_provider_prefix() and warn messages

Test plan

• cargo build --workspace --lib compiles
• cargo test --workspace — existing tests pass (no signature changes to public API)
• Start daemon with [default_model] provider = "ollama" and an agent with [[fallback_models]] provider = "default" — verify the Unknown provider 'default' warning is gone
• Kill the primary LLM endpoint mid-request — verify the fallback driver activates using the resolved provider

🤖 Generated with Claude Code

[jaberjaber23]

Two bugs in the implementation:

1. base_url leaks across providers. The PR unconditionally inherits dm.base_url for all fallback providers via .or_else(|| dm.base_url.clone()). But the primary model logic explicitly guards this with if agent_provider == default_provider (with comment "Don't inherit default provider's base_url when switching providers"). If default is ollama with base_url = "http://localhost:11434", an openai fallback would incorrectly get ollama's URL. Must guard with fb_provider == dm.provider.

2. Ignores hot-reload. The PR uses &self.config.default_model directly, but primary resolution reads self.default_model_override first (the hot-reload mechanism for dashboard model changes). Should use effective_default already computed at line 4614.

3. Dead let _ = &fb_model_name; line — does nothing, fb_model_name is already used on the next line.

4. Zero tests added for a kernel-level driver init change. Need at minimum: "default" provider resolves correctly, cross-provider base_url doesn't leak, empty strings resolve to default.

The fix direction is correct but needs these issues addressed.