fix: deep focus mode — single-problem context for Timmy (#409)

Co-authored-by: Kimi Agent <kimi@timmy.local>
Co-committed-by: Kimi Agent <kimi@timmy.local>

src/timmy/cli.py

@@ -416,5 +416,40 @@ def route(
         typer.echo("→ orchestrator (no pattern match)")
 
 
+@app.command()
+def focus(
+    topic: str | None = typer.Argument(
+        None, help='Topic to focus on (e.g. "three-phase loop"). Omit to show current focus.'
+    ),
+    clear: bool = typer.Option(False, "--clear", "-c", help="Clear focus and return to broad mode"),
+):
+    """Set deep-focus mode on a single problem.
+
+    When focused, Timmy prioritizes the active topic in all responses
+    and deprioritizes unrelated context. Focus persists across sessions.
+
+    Examples:
+        timmy focus "three-phase loop"   # activate deep focus
+        timmy focus                       # show current focus
+        timmy focus --clear               # return to broad mode
+    """
+    from timmy.focus import focus_manager
+
+    if clear:
+        focus_manager.clear()
+        typer.echo("Focus cleared — back to broad mode.")
+        return
+
+    if topic:
+        focus_manager.set_topic(topic)
+        typer.echo(f'Deep focus activated: "{topic}"')
+    else:
+        # Show current focus status
+        if focus_manager.is_focused():
+            typer.echo(f'Deep focus: "{focus_manager.get_topic()}"')
+        else:
+            typer.echo("No active focus (broad mode).")
+
+
 def main():
     app()