feat: Enhance update checker with async checking and better UX

Implemented key improvements from rxiv-maker's update checker:

- Async background checking (non-blocking, eliminates CLI startup latency)
- Environment opt-out variables (TASKREPO_NO_UPDATE_CHECK, NO_UPDATE_NOTIFIER)
- Richer cache structure (update_available, latest_version, current_version)
- Singleton pattern for consistent state
- Enhanced notifications with installation method info and GitHub links
- Force check via tsk upgrade --check

All 152 unit tests pass.

Author: paxcalpt

src/taskrepo/cli/commands/upgrade.py

@@ -8,7 +8,7 @@
 
 from taskrepo.__version__ import __version__
 from taskrepo.utils.install_detector import detect_install_method
-from taskrepo.utils.update_checker import check_for_updates
+from taskrepo.utils.update_checker import force_update_check
 
 
 def detect_installer() -> Tuple[str, list[str]]:
@@ -102,11 +102,11 @@ def upgrade(ctx, check, yes):
     """
     # Check for updates
     click.echo("Checking for updates...")
-    latest_version = check_for_updates()
+    update_available, latest_version = force_update_check()
 
     if check:
         # Just show version information
-        if latest_version:
+        if update_available and latest_version:
             click.echo(f"Current version: v{__version__}")
             click.secho(f"Latest version: v{latest_version}", fg="green", bold=True)
             click.secho("Update available!", fg="yellow")
@@ -116,7 +116,7 @@ def upgrade(ctx, check, yes):
         return
 
     # No update available
-    if not latest_version:
+    if not update_available or not latest_version:
         click.secho(f"✓ You are already using the latest version (v{__version__})", fg="green")
         return
 

src/taskrepo/cli/main.py

@@ -23,7 +23,7 @@
 from taskrepo.cli.commands.upgrade import upgrade
 from taskrepo.core.config import Config
 from taskrepo.utils.banner import display_banner
-from taskrepo.utils.update_checker import check_and_notify_updates
+from taskrepo.utils.update_checker import check_and_notify_updates, show_update_notification
 
 
 class OrderedGroup(click.Group):
@@ -133,9 +133,12 @@ def process_result(ctx, result, **kwargs):
 
     This runs after any command completes and checks for updates.
     """
-    # Check for updates after command completes
+    # Start async update check in background (if due)
     check_and_notify_updates()
 
+    # Show any cached update notifications immediately
+    show_update_notification()
+
 
 # Register commands
 cli.add_command(add)