Add POSIX compatibility spec and acceptance tests for TruShell v3

[exlier]

Related to discussion #75
Closes issue #80

Summary by CodeRabbit

• New Features

  • Improved shell command handling with POSIX-style quoting and backslash escaping.
  • cd now supports changing to the home directory when used without arguments.
  • External commands correctly receive quoted arguments as single values.
  • Unrecognized input can fall back to external command execution.

• Bug Fixes

  • Improved interactive handling of parsed and unparsed commands.

• Documentation

  • Added compatibility expectations and acceptance criteria for POSIX-oriented shell behavior.