docs: add installer agent guidance

[pablopunk]

This PR adds an AGENTS.md file containing guidance for working on the installer, shell, and terminal features of the dot project. The new documentation outlines:

• How to preserve shell data‑flow semantics (e.g., not rewriting or stealing stdin in pipelines).
• Rules for interactive vs. non‑interactive execution when a controlling terminal does or doesn’t exist.
• Requirements for regression tests covering pipeline input, shell redirection, terminal fallback, and platform‑specific process launchers.
• Concrete instructions for reproducing and testing changes using a pseudo‑terminal and the compiled binary.
• Verification commands to run focused tests and builds.

The change is purely documentation and helps contributors (both human and automated agents) maintain consistency and correctness when modifying bootstrap, command execution, TTY/stdin handling, and installer ordering.

[kody-ai]

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