Fix shell integration status detection

[jestenough]

Summary

• Distinguish shell-wrapper invocation from the navigation protocol.
• Add the PCD_WRAPPER marker for Bash, Zsh, and Fish.
• Keep PCD_SHELL dedicated to directory navigation.
• Update pcd shell status output.
• Ensure the wrapper marker is scoped to a single command invocation.
• Add documentation and integration/E2E coverage.

Behavior

Direct invocation:

Invoked through wrapper: no

Invocation through the shell wrapper:

Invoked through wrapper: yes (bash)