feat(cli): add ansi_basic theme fallback and offline provider shim for limited‑color terminals

- Detect terminal color support and automatically switch to the new `ansi_basic` theme when only 16‑color support is available.
- Introduce `OfflineProvider` that supplies a placeholder model and friendly messages when no providers are reachable, keeping the TUI usable.
- Add `CONFIG_SCHEMA_VERSION` (`1.1.0`) with schema migration logic and default handling in `Config`.
- Update configuration saving to persist the schema version and ensure defaults.
- Register the `ansi_basic` theme in `theme.rs`.
- Extend `ChatApp` with `set_status_message` to display custom status lines.
- Update documentation (architecture, Vim mode state machine) to reflect new behavior.
- Add async‑trait and futures dependencies required for the offline provider implementation.

crates/owlen-core/src/config.rs

@@ -10,9 +10,15 @@ use std::time::Duration;
 /// Default location for the OWLEN configuration file
 pub const DEFAULT_CONFIG_PATH: &str = "~/.config/owlen/config.toml";
 
+/// Current schema version written to `config.toml`.
+pub const CONFIG_SCHEMA_VERSION: &str = "1.1.0";
+
 /// Core configuration shared by all OWLEN clients
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct Config {
+    /// Schema version for on-disk configuration files
+    #[serde(default = "Config::default_schema_version")]
+    pub schema_version: String,
     /// General application settings
     pub general: GeneralSettings,
     /// MCP (Multi-Client-Provider) settings
@@ -57,6 +63,7 @@ impl Default for Config {
         );
 
         Self {
+            schema_version: Self::default_schema_version(),
             general: GeneralSettings::default(),
             mcp: McpSettings::default(),
             providers,
@@ -97,6 +104,10 @@ impl McpServerConfig {
 }
 
 impl Config {
+    fn default_schema_version() -> String {
+        CONFIG_SCHEMA_VERSION.to_string()
+    }
+
     /// Load configuration from disk, falling back to defaults when missing
     pub fn load(path: Option<&Path>) -> Result<Self> {
         let path = match path {
@@ -106,10 +117,27 @@ impl Config {
 
         if path.exists() {
             let content = fs::read_to_string(&path)?;
-            let mut config: Config =
+            let parsed: toml::Value =
                 toml::from_str(&content).map_err(|e| crate::Error::Config(e.to_string()))?;
+            let previous_version = parsed
+                .get("schema_version")
+                .and_then(|value| value.as_str())
+                .unwrap_or("0.0.0")
+                .to_string();
+            if let Some(agent_table) = parsed.get("agent").and_then(|value| value.as_table()) {
+                if agent_table.contains_key("max_tool_calls") {
+                    log::warn!(
+                        "Configuration option agent.max_tool_calls is deprecated and ignored. \
+                         The agent now uses agent.max_iterations."
+                    );
+                }
+            }
+            let mut config: Config = parsed
+                .try_into()
+                .map_err(|e: toml::de::Error| crate::Error::Config(e.to_string()))?;
             config.ensure_defaults();
             config.mcp.apply_backward_compat();
+            config.apply_schema_migrations(&previous_version);
             config.validate()?;
             Ok(config)
         } else {
@@ -130,8 +158,10 @@ impl Config {
             fs::create_dir_all(dir)?;
         }
 
+        let mut snapshot = self.clone();
+        snapshot.schema_version = Config::default_schema_version();
         let content =
-            toml::to_string_pretty(self).map_err(|e| crate::Error::Config(e.to_string()))?;
+            toml::to_string_pretty(&snapshot).map_err(|e| crate::Error::Config(e.to_string()))?;
         fs::write(path, content)?;
         Ok(())
     }
@@ -171,6 +201,9 @@ impl Config {
 
         ensure_provider_config(self, "ollama");
         ensure_provider_config(self, "ollama-cloud");
+        if self.schema_version.is_empty() {
+            self.schema_version = Self::default_schema_version();
+        }
     }
 
     /// Validate configuration invariants and surface actionable error messages.
@@ -181,6 +214,17 @@ impl Config {
         Ok(())
     }
 
+    fn apply_schema_migrations(&mut self, previous_version: &str) {
+        if previous_version != CONFIG_SCHEMA_VERSION {
+            log::info!(
+                "Upgrading configuration schema from '{}' to '{}'",
+                previous_version,
+                CONFIG_SCHEMA_VERSION
+            );
+        }
+        self.schema_version = CONFIG_SCHEMA_VERSION.to_string();
+    }
+
     fn validate_default_provider(&self) -> Result<()> {
         if self.general.default_provider.trim().is_empty() {
             return Err(crate::Error::Config(