fix(envd): clean up error formatting when watch path is not a directory

[chill-czar]

Closes #3581

Summary

• Remove %w formatting verb on nil error in packages/envd/internal/services/filesystem/watch.go and watch_sync.go when the requested watch path is not a directory.
• Return clean error message: path <path> is not a directory instead of path <path> not a directory: %!w(<nil>).
• Add unit test TestCreateWatcher_NotADirectory in packages/envd/internal/services/filesystem/watch_test.go asserting clean error strings without %!w.

Why

In watch.go and watch_sync.go, when os.Stat succeeds for a non-directory path, err is nil. Calling fmt.Errorf("path %s not a directory: %w", watchPath, err) resulted in %!w(<nil>) being appended to the error string returned across the Connect-RPC wire and emitted in server logs.

Diff Overview

 	if !info.IsDir() {
-		return connect.NewError(connect.CodeInvalidArgument, fmt.Errorf("path %s not a directory: %w", watchPath, err))
+		return connect.NewError(connect.CodeInvalidArgument, fmt.Errorf("path %s is not a directory", watchPath))
 	}

Test Plan

• Unit test: go test -v ./packages/envd/internal/services/filesystem -run TestCreateWatcher_NotADirectory
• Package test suite passes: go test -v ./packages/envd/internal/services/filesystem/...
• Linter & Formatter pass: make fmt and make lint
• Verified error output contains "is not a directory" and does not contain "%!w"

/cc @jakubno @dobrac @ValentaTomas @arkamar @tvi