backend: move default backend selection to backends.conf

backends.conf now supports a top-level 'backend = name' line that
selects the default backend for new sessions. Lookup order:

  1. backends.conf 'backend = ...'
  2. OLLIE_BACKEND env var (fallback)
  3. session-level backend= parameter (override)

No env file needed for backend/model config anymore.

backend/config.go

@@ -17,10 +17,13 @@ type BackendConfig struct {
 }
 
 // configFile is the parsed contents of backends.conf: section name → key/value pairs.
-type configFile map[string]BackendConfig
+type configFile struct {
+	Default  string // top-level "backend = ..." selects the default
+	Backends map[string]BackendConfig
+}
 
 // loadConfig reads ~/.config/ollie/backends.conf.
-// Returns an empty map (not nil) if the file doesn't exist or is unreadable.
+// Returns zero value if the file doesn't exist or is unreadable.
 func loadConfig() configFile {
 	return parseConfigFile(paths.CfgDir() + "/backends.conf")
 }
@@ -28,6 +31,8 @@ func loadConfig() configFile {
 // parseConfigFile parses an INI-style config file.
 // Format:
 //
+//	backend = openrouter          (top-level: default backend selection)
+//
 //	[section]
 //	key = value
 //
@@ -35,11 +40,11 @@ func loadConfig() configFile {
 func parseConfigFile(path string) configFile {
 	f, err := os.Open(path)
 	if err != nil {
-		return configFile{}
+		return configFile{Backends: map[string]BackendConfig{}}
 	}
 	defer f.Close()
 
-	cfg := configFile{}
+	cfg := configFile{Backends: map[string]BackendConfig{}}
 	section := ""
 
 	sc := bufio.NewScanner(f)
@@ -51,15 +56,12 @@ func parseConfigFile(path string) configFile {
 		// Section header.
 		if strings.HasPrefix(line, "[") && strings.HasSuffix(line, "]") {
 			section = strings.TrimSpace(line[1 : len(line)-1])
-			if _, ok := cfg[section]; !ok {
-				cfg[section] = BackendConfig{}
+			if _, ok := cfg.Backends[section]; !ok {
+				cfg.Backends[section] = BackendConfig{}
 			}
 			continue
 		}
 		// Key = value.
-		if section == "" {
-			continue // ignore keys before first section
-		}
 		k, v, ok := strings.Cut(line, "=")
 		if !ok {
 			continue
@@ -71,7 +73,15 @@ func parseConfigFile(path string) configFile {
 			v = v[1 : len(v)-1]
 		}
 
-		bc := cfg[section]
+		// Top-level keys (before any section).
+		if section == "" {
+			if k == "backend" {
+				cfg.Default = v
+			}
+			continue
+		}
+
+		bc := cfg.Backends[section]
 		switch k {
 		case "key":
 			bc.Key = v
@@ -82,7 +92,7 @@ func parseConfigFile(path string) configFile {
 		case "host":
 			bc.Host = v
 		}
-		cfg[section] = bc
+		cfg.Backends[section] = bc
 	}
 	return cfg
 }

backend/new.go

@@ -6,20 +6,27 @@ import (
 )
 
 // NewWithName constructs a Backend for the given backend name.
-// If name is empty, falls back to OLLIE_BACKEND; errors if neither is set.
+// If name is empty, falls back to backends.conf default, then OLLIE_BACKEND env.
 func NewWithName(name string) (Backend, error) {
+	cfg := loadConfig()
+	if name == "" {
+		name = cfg.Default
+	}
 	if name == "" {
 		name = os.Getenv("OLLIE_BACKEND")
-		if name == "" {
-			return nil, fmt.Errorf("no backend specified: set OLLIE_BACKEND or pass backend= to the session")
-		}
 	}
-	return newBackend(name)
+	if name == "" {
+		return nil, fmt.Errorf("no backend specified: set 'backend' in backends.conf, OLLIE_BACKEND, or pass backend= to the session")
+	}
+	return newBackendFromConfig(name, cfg)
 }
 
 func newBackend(which string) (Backend, error) {
-	cfg := loadConfig()
-	bc := cfg[which]
+	return newBackendFromConfig(which, loadConfig())
+}
+
+func newBackendFromConfig(which string, cfg configFile) (Backend, error) {
+	bc := cfg.Backends[which]
 
 	switch which {
 	case "ollama":

data/backends.conf

@@ -1,13 +1,18 @@
 # Ollie backend configuration.
 # Each section configures a backend. The session selects which backend to use
-# via OLLIE_BACKEND env var or backend= parameter.
+# via the top-level 'backend' setting or backend= parameter.
 #
-# Fields:
+# Top-level:
+#   backend = name              (default backend for new sessions)
+#
+# Per-backend fields:
 #   key   = API key or token
 #   url   = base URL override (optional; each backend has a sensible default)
 #   model = default model (optional; overridden by session model= parameter)
 #   host  = host address (ollama only)
 
+backend = openrouter
+
 [openrouter]
 # key = sk-or-v1-...
 # model = qwen/qwen3-235b-a22b