feat: add automatic greeting upon connection (#42)

### Explanation

_No response_

### Issue

Closes #23

### Attestations

- [ ] I have read and agree to the [Code of Conduct](https://docs.nhcarrigan.com/community/coc/)
- [ ] I have read and agree to the [Community Guidelines](https://docs.nhcarrigan.com/community/guide/).
- [ ] My contribution complies with the [Contributor Covenant](https://docs.nhcarrigan.com/dev/covenant/).

### Dependencies

- [ ] I have pinned the dependencies to a specific patch version.

### Style

- [ ] I have run the linter and resolved any errors.
- [ ] My pull request uses an appropriate title, matching the conventional commit standards.
- [ ] My scope of feat/fix/chore/etc. correctly matches the nature of changes in my pull request.

### Tests

- [ ] My contribution adds new code, and I have added tests to cover it.
- [ ] My contribution modifies existing code, and I have updated the tests to reflect these changes.
- [ ] All new and existing tests pass locally with my changes.
- [ ] Code coverage remains at or above the configured threshold.

### Documentation

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### Versioning

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Reviewed-on: #42
Co-authored-by: Naomi Carrigan <commits@nhcarrigan.com>
Co-committed-by: Naomi Carrigan <commits@nhcarrigan.com>

src-tauri/src/config.rs

@@ -21,7 +21,7 @@ pub struct ClaudeStartOptions {
     pub allowed_tools: Vec<String>,
 }
 
-#[derive(Debug, Clone, Serialize, Deserialize, Default)]
+#[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct HikariConfig {
     #[serde(default)]
     pub model: Option<String>,
@@ -40,6 +40,31 @@ pub struct HikariConfig {
 
     #[serde(default)]
     pub theme: Theme,
+
+    #[serde(default = "default_greeting_enabled")]
+    pub greeting_enabled: bool,
+
+    #[serde(default)]
+    pub greeting_custom_prompt: Option<String>,
+}
+
+impl Default for HikariConfig {
+    fn default() -> Self {
+        Self {
+            model: None,
+            api_key: None,
+            custom_instructions: None,
+            mcp_servers_json: None,
+            auto_granted_tools: Vec::new(),
+            theme: Theme::default(),
+            greeting_enabled: true,
+            greeting_custom_prompt: None,
+        }
+    }
+}
+
+fn default_greeting_enabled() -> bool {
+    true
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize, Default, PartialEq)]
@@ -63,6 +88,8 @@ mod tests {
         assert!(config.mcp_servers_json.is_none());
         assert!(config.auto_granted_tools.is_empty());
         assert_eq!(config.theme, Theme::Dark);
+        assert!(config.greeting_enabled);
+        assert!(config.greeting_custom_prompt.is_none());
     }
 
     #[test]
@@ -74,6 +101,8 @@ mod tests {
             mcp_servers_json: None,
             auto_granted_tools: vec!["Read".to_string(), "Glob".to_string()],
             theme: Theme::Light,
+            greeting_enabled: true,
+            greeting_custom_prompt: Some("Hello!".to_string()),
         };
 
         let json = serde_json::to_string(&config).unwrap();
@@ -83,6 +112,8 @@ mod tests {
         assert_eq!(deserialized.custom_instructions, config.custom_instructions);
         assert_eq!(deserialized.auto_granted_tools, config.auto_granted_tools);
         assert_eq!(deserialized.theme, Theme::Light);
+        assert!(deserialized.greeting_enabled);
+        assert_eq!(deserialized.greeting_custom_prompt, Some("Hello!".to_string()));
     }
 
     #[test]