feat: initial Sol virtual librarian implementation

Matrix bot with E2EE (matrix-sdk 0.9) that passively archives all
messages to OpenSearch and responds to queries via Mistral AI with
function calling tools.

Core systems:
- Archive: bulk OpenSearch indexer with batch/flush, edit/redaction
  handling, embedding pipeline passthrough
- Brain: rule-based engagement evaluator (mentions, DMs, name
  invocations), LLM-powered spontaneous engagement, per-room
  conversation context windows, response delay simulation
- Tools: search_archive, get_room_context, list_rooms, get_room_members
  registered as Mistral function calling tools with iterative tool loop
- Personality: templated system prompt with Sol's librarian persona

47 unit tests covering config, evaluator, conversation windowing,
personality templates, schema serialization, and search query building.

src/brain/personality.rs

@@ -0,0 +1,89 @@
+use chrono::Utc;
+
+pub struct Personality {
+    template: String,
+}
+
+impl Personality {
+    pub fn new(system_prompt: String) -> Self {
+        Self {
+            template: system_prompt,
+        }
+    }
+
+    pub fn build_system_prompt(
+        &self,
+        room_name: &str,
+        members: &[String],
+        ) -> String {
+        let date = Utc::now().format("%Y-%m-%d").to_string();
+        let members_str = members.join(", ");
+
+        self.template
+            .replace("{date}", &date)
+            .replace("{room_name}", room_name)
+            .replace("{members}", &members_str)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_date_substitution() {
+        let p = Personality::new("Today is {date}.".to_string());
+        let result = p.build_system_prompt("general", &[]);
+        let today = Utc::now().format("%Y-%m-%d").to_string();
+        assert_eq!(result, format!("Today is {today}."));
+    }
+
+    #[test]
+    fn test_room_name_substitution() {
+        let p = Personality::new("You are in {room_name}.".to_string());
+        let result = p.build_system_prompt("design-chat", &[]);
+        assert!(result.contains("design-chat"));
+    }
+
+    #[test]
+    fn test_members_substitution() {
+        let p = Personality::new("Members: {members}".to_string());
+        let members = vec!["Alice".to_string(), "Bob".to_string(), "Carol".to_string()];
+        let result = p.build_system_prompt("room", &members);
+        assert_eq!(result, "Members: Alice, Bob, Carol");
+    }
+
+    #[test]
+    fn test_empty_members() {
+        let p = Personality::new("Members: {members}".to_string());
+        let result = p.build_system_prompt("room", &[]);
+        assert_eq!(result, "Members: ");
+    }
+
+    #[test]
+    fn test_all_placeholders() {
+        let template = "Date: {date}, Room: {room_name}, People: {members}".to_string();
+        let p = Personality::new(template);
+        let members = vec!["Sienna".to_string(), "Lonni".to_string()];
+        let result = p.build_system_prompt("studio", &members);
+
+        let today = Utc::now().format("%Y-%m-%d").to_string();
+        assert!(result.starts_with(&format!("Date: {today}")));
+        assert!(result.contains("Room: studio"));
+        assert!(result.contains("People: Sienna, Lonni"));
+    }
+
+    #[test]
+    fn test_no_placeholders_passthrough() {
+        let p = Personality::new("Static prompt with no variables.".to_string());
+        let result = p.build_system_prompt("room", &["Alice".to_string()]);
+        assert_eq!(result, "Static prompt with no variables.");
+    }
+
+    #[test]
+    fn test_multiple_same_placeholder() {
+        let p = Personality::new("{room_name} is great. I love {room_name}.".to_string());
+        let result = p.build_system_prompt("lounge", &[]);
+        assert_eq!(result, "lounge is great. I love lounge.");
+    }
+}