feat: pty

[cmilesdev]

• Added WithPTY() to run commands on a PTY so tools still detect a TTY while OnStdout/OnStderr callbacks are used; output is combined in PTY mode.
• Preserved TTY passthrough when writers are real terminals and no callbacks/combined output are enabled.
• Exposed OnExecCmd hook to mutate the underlying exec.Cmd before start.
• Added Linux/Darwin PTY implementations with no external deps and comprehensive tests to keep 100% coverage.
• Updated docs and examples for new APIs and behavior changes.

Behavior Changes

• When WithPTY() is enabled, stdout/stderr are merged; OnStdout and OnStderr both receive the same lines; Result.Stderr is empty.
• WithPTY() is not supported for pipelines and returns a clear error.
• When Stdout/Stderr writers are real terminals and no callbacks/combined output are used, execx now passes through without buffering.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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