Add contact name filter to SMS search tool + misc improvements

- sms search tool: accept contact name, trim/validate, skip when
  contact_id is set, pass to API client
- sms_client: new contact field in SmsSearchParams, URL-encode on wire
- Tool description clarifies contact_id takes precedence when both given
- Add parse_title_body helper for LLM response parsing
- llamacpp backend improvements

src/ai/sms_client.rs

@@ -281,6 +281,9 @@ impl SmsApiClient {
         if let Some(cid) = params.contact_id {
             url.push_str(&format!("&contact_id={}", cid));
         }
+        if let Some(ref c) = params.contact {
+            url.push_str(&format!("&contact={}", urlencoding::encode(c)));
+        }
         if let Some(off) = params.offset {
             url.push_str(&format!("&offset={}", off));
         }
@@ -413,6 +416,9 @@ pub struct SmsSearchParams<'a> {
     pub mode: &'a str,
     pub limit: usize,
     pub contact_id: Option<i64>,
+    /// Contact name (case-insensitive). Resolved to a numeric ID by the
+    /// SMS-API server when `contact_id` is not set.
+    pub contact: Option<String>,
     /// Unix-seconds inclusive lower bound on `date`.
     pub date_from: Option<i64>,
     /// Unix-seconds inclusive upper bound on `date`.