config: unify subconscious API resolution with the main chat path

Two parallel backend-resolution paths had drifted apart:

- Main chat: AppConfig::resolve_model() → a named BackendConfig in
  AppConfig.backends
- Subconscious / oneshot / context_window(): four skip-serde
  "cache" fields on Config (memory section) — api_base_url, api_key,
  api_model, api_context_window — that used to be populated at
  Config::try_load_shared time by walking memory.agent_model →
  root.models[name] → root[backend_name]

When we renamed `models` to `backends` and collapsed ModelConfig into
BackendConfig, the latter chain started silently dereferencing
`root.get("models")` → None → no population. Subconscious agents fell
through the "API not configured" guard; context_window() started
returning 0 (since api_context_window default is u64's 0 now that we
don't populate it). It was only visibly working for the main chat.

Collapse to one path:

- Drop Config.agent_model (duplicate of AppConfig.default_backend)
- Drop Config.{api_base_url, api_key, api_model, api_context_window}
  — no longer populated, no longer needed
- Drop default_context_window() — nobody reads the field anymore
- Drop the memory-side resolution block in try_load_shared()
- Subconscious (mind/unconscious.rs) and oneshot (agent/oneshot.rs)
  now call load_app() + resolve_model(&app.default_backend) just like
  the main chat does
- context_window() reads from config::app().backends[default_backend]
  .context_window, defaulting to 128k only if the backend doesn't
  specify one

Side effect: Kent's config file drops agent_model, api_reasoning,
journal_days, journal_max — all fields whose Rust counterparts are
now gone. (Figment tolerates unknown fields, so leaving them wouldn't
have broken anything, but they were lying about what's configurable.)

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/agent/context.rs

@@ -992,7 +992,10 @@ impl ContextState {
 }
 
 pub fn context_window() -> usize {
-    crate::config::get().api_context_window
+    let app = crate::config::app();
+    app.backends.get(&app.default_backend)
+        .and_then(|b| b.context_window)
+        .unwrap_or(128_000)
 }
 
 pub fn context_budget_tokens() -> usize {