fix(envd): malformed error message %!w(<nil>) when watch path is not a directory

Problem

In packages/envd/internal/services/filesystem/watch.go and watch_sync.go, the guest agent's filesystem service checks whether a path targeted for watching is a valid directory:

info, err := os.Stat(watchPath)
if err != nil {
    if os.IsNotExist(err) {
        return connect.NewError(connect.CodeNotFound, fmt.Errorf("path %s not found: %w", watchPath, err))
    }
    return connect.NewError(connect.CodeInternal, fmt.Errorf("error statting path %s: %w", watchPath, err))
}

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

When os.Stat(watchPath) succeeds for a non-directory (e.g. a regular file, FIFO, or socket), err is nil. Because err is nil, Go's fmt.Errorf %w verb produces the malformed string %!w(<nil>).

Clients and Connect-RPC error logs receive corrupted messages such as:

path /home/user/file.txt not a directory: %!w(<nil>)

Root Cause

fmt.Errorf was invoked with the error-wrapping format verb %w against a nil error reference instead of formatting a clean static error message:

This formatting also aligns with other filesystem endpoints in packages/envd/internal/services/filesystem/dir.go:L131 ("path is not a directory: %s").

Reproduction Steps

1. Start envd.
2. Create a normal file /home/user/test.txt.
3. Call CreateWatcher on /home/user/test.txt.
4. Observed Error: connect.CodeInvalidArgument with message "path /home/user/test.txt not a directory: %!w(<nil>)".
5. Expected Error: connect.CodeInvalidArgument with message "path /home/user/test.txt is not a directory".

Technical Context

• Files affected: packages/envd/internal/services/filesystem/watch.go, packages/envd/internal/services/filesystem/watch_sync.go
• Subsystem: Guest Agent (envd) / Filesystem Service
• Impact: Low (Error cleanliness, log readability, and API response quality)

Proposed Changes

Start in packages/envd/internal/services/filesystem/watch.go and watch_sync.go, focusing on the non-directory checks described in the issue. Add or update the TestCreateWatcher_NotADirectory test in watch_test.go, then run the filesystem service tests. Done means both watcher paths return a clean invalid-argument message without %!w when given a non-directory.