config: drop dead code and collapse to a single backend

Config had accumulated several obsolete fields, a legacy load path
that was just returning defaults, and multi-backend infrastructure
that's no longer used.

Removed from Config (memory section):
- load_legacy_jsonl() — just returned Config::default(), no callers
- The legacy-fallback branch in load_from_file
- surface_hooks, surface_timeout_secs — zero external readers
- scoring_chunk_tokens + default fn — zero external readers
- The POC_MEMORY_CONFIG env override note in the header comment
  (not actually wired up anywhere)

Collapsed multi-backend to single-backend:
- AppConfig used to carry `anthropic: BackendConfig` and
  `openrouter: BackendConfig` as required fields plus an optional
  `deepinfra`, picked between at runtime by name. Only one is ever
  actually used in any deployment. Collapse to a single
  `backend: BackendConfig` on AppConfig, drop the multi-backend
  match logic in resolve_model, drop the top-level `backend: String`
  selector field, drop the `BackendConfig::resolve` fallback path.
- Also drop BackendConfig.model (redundant with ModelConfig.model_id
  once multi-backend is gone).
- ModelConfig.backend field goes — there's only one backend now, no
  choice to make.

Dead prompt_file machinery:
- ModelConfig.prompt_file, ResolvedModel.prompt_file, SessionConfig
  .prompt_file, Agent.prompt_file — nothing in the codebase actually
  reads the file these strings name. Just passed around and compared.
  Delete the whole string through every struct.
- The "if prompt_file changed on model switch, recompact" branch in
  user/chat.rs goes too (never fired usefully).

Dead memory_project plumbing:
- AppConfig.memory_project field, CliArgs.memory_project, the
  --memory-project CLI flag, the figment merge target, the show_config
  display line. Nothing reads it anywhere.

Dead ContextInfo struct:
- `struct ContextInfo` was never constructed — context_info: None
  was the only initializer. The conditional display blocks in
  user/context.rs that dereferenced it were dead.

Behavior change: AppConfig::resolve() now requires a non-empty
`models` map and bails with a helpful message if it's missing. The
old fallback ("no models? use top-level backend + PromptConfig to
build a default") path is gone — it was only kept for symmetry with
a mode nobody used.

Config file shape: `deepinfra: {...}` → `backend: {...}`, and
model entries no longer need `backend:` or `prompt_file:`. Updated
~/.consciousness/config.json5 to match.

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

src/agent/mod.rs

@@ -139,7 +139,6 @@ impl DispatchState {
 pub struct Agent {
     pub client: ApiClient,
     pub app_config: crate::config::AppConfig,
-    pub prompt_file: String,
     pub session_id: String,
     pub context: crate::Mutex<ContextState>,
     pub state: crate::Mutex<AgentState>,
@@ -189,7 +188,6 @@ impl Agent {
         client: ApiClient,
         personality: Vec<(String, String)>,
         app_config: crate::config::AppConfig,
-        prompt_file: String,
         conversation_log: Option<ConversationLog>,
         active_tools: tools::ActiveTools,
         agent_tools: Vec<tools::Tool>,
@@ -220,7 +218,6 @@ impl Agent {
         let agent = Arc::new(Self {
             client,
             app_config,
-            prompt_file,
             session_id,
             context: crate::Mutex::new(context),
             state: crate::Mutex::new(AgentState {
@@ -259,7 +256,6 @@ impl Agent {
         Arc::new(Self {
             client: self.client.clone(),
             app_config: self.app_config.clone(),
-            prompt_file: self.prompt_file.clone(),
             session_id: self.session_id.clone(),
             context: crate::Mutex::new(ctx),
             state: crate::Mutex::new(AgentState {