feat: externalize LLM providers and tool enable flags via config

Adds ProviderConfig + providers list and per-tool enabled flags to
CorrexConfig, and rewires apps/server/Main to build the provider list
from config instead of the hardcoded buildLlamaProvider() factory. Env
vars (CORREX_MODEL_ID, CORREX_MODEL_PATH, CORREX_LLAMA_URL) remain a
fallback only when no providers are declared in config.

Completes slice A of the config layer alongside commit 0834c70 which
already shipped the TOML parser upgrade and sample config — those
parser changes referenced these types but were committed prematurely
in isolation; this commit makes HEAD buildable.

core/config/src/main/kotlin/com/correx/core/config/CorrexConfig.kt

@@ -9,6 +9,7 @@ data class CorrexConfig(
     val cli: CliConfig = CliConfig(),
     val approval: ApprovalConfig = ApprovalConfig(),
     val tools: ToolsConfig = ToolsConfig(),
+    val providers: List<ProviderConfig> = emptyList(),
 )
 
 @Serializable
@@ -39,4 +40,18 @@ data class ToolsConfig(
     val workingDir: String = "",
     val shellAllowedExecutables: List<String> = emptyList(),
     val defaultSystemPromptPath: String = "~/.config/correx/prompts/default_system.md",
+    val shellEnabled: Boolean = true,
+    val fileReadEnabled: Boolean = true,
+    val fileWriteEnabled: Boolean = true,
+    val fileEditEnabled: Boolean = true,
+)
+
+@Serializable
+data class ProviderConfig(
+    val id: String,
+    val type: String,
+    val modelId: String,
+    val modelPath: String = "",
+    val url: String = "http://127.0.0.1:10000",
+    val capabilities: Map<String, Double> = emptyMap(),
 )