feat(playground): add local playground dev server command and skill

Add `build_sharpy playground` command (with --release and --no-watch flags)
and `/playground` Claude Code skill to run the Blazor WASM playground locally
with hot reload.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: antonsynd

.claude/skills/playground/SKILL.md

@@ -0,0 +1,26 @@
+---
+name: playground
+description: Run the Sharpy playground (Blazor WASM) locally with hot reload
+---
+
+Run the Sharpy playground locally as a Blazor WebAssembly dev server with hot reload.
+
+**Usage:** `/playground`
+
+**Behavior:**
+- Builds and serves `src/Sharpy.Playground` with `dotnet watch run`
+- Opens at `http://localhost:5000` or `https://localhost:5001`
+- Hot reloads on file changes (Playground, Compiler, Core)
+- Ctrl+C to stop
+
+**Log location:** `.claude/tmp/last-playground.log`
+
+## Steps
+
+1. Run `mkdir -p .claude/tmp` to ensure log directory exists
+2. Clear the old log with `rm -f .claude/tmp/last-playground.log`
+3. Tell the user: "Starting playground at http://localhost:5000 (https://localhost:5001). Press Ctrl+C to stop."
+4. Run: `dotnet watch run --project src/Sharpy.Playground > .claude/tmp/last-playground.log 2>&1`
+   - **Important:** This is a long-running process. Run it in the background with `run_in_background: true` so the user can continue working.
+5. After launching, tell the user the playground is running and they can open `http://localhost:5000` in their browser.
+6. If the process fails immediately (exit code non-zero within a few seconds): Print "=== PLAYGROUND FAILED ===" then `tail -50 .claude/tmp/last-playground.log`, then echo "=== Full log: .claude/tmp/last-playground.log ==="

build_tools/bin/build_sharpy

@@ -23,6 +23,8 @@ CORE_DIR: Path = SRC_DIR / "Sharpy.Core"
 CORE_TEST_DIR: Path = SRC_DIR / "Sharpy.Core.Tests"
 CLI_DIR: Path = SRC_DIR / "Sharpy.Cli"
 
+PLAYGROUND_DIR: Path = SRC_DIR / "Sharpy.Playground"
+
 EDITORS_DIR: Path = ROOT_DIR / "editors"
 VSCODE_EXT_DIR: Path = EDITORS_DIR / "vscode"
 
@@ -84,6 +86,8 @@ def main() -> None:
         install(args=args, rest=rest)
     elif command == "test":
         test(args=args, rest=rest, release=False)
+    elif command == "playground":
+        playground(args=args, rest=rest)
     elif command == "help":
         parser.print_help()
     else:
@@ -156,6 +160,16 @@ def create_parser() -> argparse.ArgumentParser:
         choices=TARGET_CONSTS,
         default=None,
     )
+    playground_parser = subparsers.add_parser(
+        "playground", help="Runs the Blazor WASM playground locally."
+    )
+    playground_parser.add_argument("--release", action="store_true")
+    playground_parser.add_argument(
+        "--no-watch",
+        action="store_true",
+        help="Run without hot reload (dotnet run instead of dotnet watch run)",
+    )
+
     _ = subparsers.add_parser("help", help="Prints this help message.")
 
     return parser
@@ -273,6 +287,32 @@ def build_tests(release: bool) -> None:
         sys.exit(1)
 
 
+def playground(args: argparse.Namespace, rest: Sequence[str]) -> None:
+    build_type: str = "Release" if args.release else "Debug"
+    project: str = str(PLAYGROUND_DIR)
+
+    get_logger().info(f"Starting playground ({build_type.lower()})...")
+    get_logger().info("Open http://localhost:5000 or https://localhost:5001 in your browser")
+    get_logger().info("Press Ctrl+C to stop")
+
+    try:
+        if args.no_watch:
+            subprocess.run(
+                ["dotnet", "run", "--project", project, "-c", build_type, *rest],
+                check=True,
+            )
+        else:
+            subprocess.run(
+                ["dotnet", "watch", "run", "--project", project, "-c", build_type, *rest],
+                check=True,
+            )
+    except subprocess.CalledProcessError as e:
+        get_logger().error(f"Playground failed: {e}")
+        sys.exit(1)
+    except KeyboardInterrupt:
+        get_logger().info("Playground stopped.")
+
+
 def setup(args: argparse.Namespace, rest: Sequence[str]) -> None:
     get_logger().info("Setting up...")
     get_logger().info("Restoring project...")