feat(cli): add thread clear-name to remove a custom thread title (1600) (#1939)

Issue 1600 asks for full rename UX: set a custom title, remove it back
to `(unnamed)`. The set side already shipped as `deepseek thread
set-name <id> <name>`, but there was no inverse — users who wanted to
drop a no-longer-relevant title had to either edit the SQLite store by
hand or set a placeholder string.

Add `deepseek thread clear-name <id>`, mirroring the existing
`SetName` arm in `run_thread_command`: look up the thread, set
`name = None`, refresh `updated_at`, upsert. Printed confirmation is
`cleared name for <id>` so it stays distinguishable from the
`renamed` line emitted by `set-name`.

Snapshot help test and the parser-matrix test both gain the new
subcommand. No state-store changes: `upsert_thread` already accepts
`name: None` and `thread list` already prints `(unnamed)` when the
field is empty, so the round-trip is complete with this CLI-only
change.

Co-authored-by: Claude <noreply@anthropic.com>

crates/cli/src/lib.rs

@@ -385,6 +385,11 @@ enum ThreadCommand {
         thread_id: String,
         name: String,
     },
+    /// Remove the custom name from a thread, restoring the default
+    /// `(unnamed)` rendering in `thread list`.
+    ClearName {
+        thread_id: String,
+    },
 }
 
 #[derive(Debug, Args)]
@@ -1258,6 +1263,16 @@ fn run_thread_command(command: ThreadCommand) -> Result<()> {
             println!("renamed {thread_id}");
             Ok(())
         }
+        ThreadCommand::ClearName { thread_id } => {
+            let mut thread = state
+                .get_thread(&thread_id)?
+                .with_context(|| format!("thread not found: {thread_id}"))?;
+            thread.name = None;
+            thread.updated_at = chrono::Utc::now().timestamp();
+            state.upsert_thread(&thread)?;
+            println!("cleared name for {thread_id}");
+            Ok(())
+        }
     }
 }
 
@@ -1925,6 +1940,14 @@ mod tests {
                 }
             })) if thread_id == "thread-6" && name == "My Thread"
         ));
+
+        let cli = parse_ok(&["deepseek", "thread", "clear-name", "thread-7"]);
+        assert!(matches!(
+            cli.command,
+            Some(Commands::Thread(ThreadArgs {
+                command: ThreadCommand::ClearName { ref thread_id }
+            })) if thread_id == "thread-7"
+        ));
     }
 
     #[test]
@@ -3048,6 +3071,7 @@ mod tests {
                     "archive",
                     "unarchive",
                     "set-name",
+                    "clear-name",
                 ],
             ),
             ("sandbox", vec!["check"]),