Use shlex to parse PAGER values

[ryanfowler]

Summary

• Parse PAGER with shlex so quoted arguments and escapes are handled correctly
• Fall back to the built-in pager when PAGER is malformed
• Update docs to clarify that fetch does not interpret shell operators in PAGER

Testing

• Added unit tests covering quoted PAGER parsing and invalid PAGER fallback
• Not run (not requested)