Add embeddedcli.Path() accessor for the resolved CLI path

[tbrady77]

What

Adds a small, additive public accessor embeddedcli.Path() that returns the absolute path to the embedded Copilot CLI (installing it on first call), wrapping the existing internal/embeddedcli.Path value.

// Path returns the absolute path to the embedded Copilot CLI, installing it on
// first call if necessary. It returns an empty string when no embedded CLI was
// configured via Setup (e.g. a build compiled without the embedded runtime).
// The result is computed once and cached for the life of the process.
func Path() string {
	return embeddedcli.Path()
}

Why

The public embeddedcli package exposes Config and Setup, but currently offers no way to read back the resolved CLI path after Setup. A consumer that embeds the CLI (e.g. a packaged desktop app shipping the runtime) needs the resolved path to spawn copilot … subcommands directly — for instance when there is no copilot on PATH in a shipped build.

The value already exists in internal/embeddedcli.Path, but internal/ isn't importable by downstream modules. This exposes it through the public surface, mirroring the existing Setup wrapper one-to-one.

Notes

• Purely additive; no behavior change to existing APIs.
• internal/embeddedcli.Path already returns "" when Setup was never called and is memoized via sync.OnceValue, so the documented semantics are inherited as-is.