docs: document TREEHOUSE_DIR and TREEHOUSE_NO_UPDATE_CHECK env vars

[e-jung]

Intent

Two environment variables are part of treehouse's public API surface but are absent from the docs:

• TREEHOUSE_DIR — exported into the treehouse get subshell (cmd/get.go:58) and read by treehouse return (cmd/return_cmd.go:82) to know which worktree to return when no path is given. Users see it in their shell after treehouse get.
• TREEHOUSE_NO_UPDATE_CHECK — set to 1 to skip the startup background update check (cmd/root.go:33). Useful in CI, automation, or air-gapped environments.

Approach

Add a concise Environment Variables section to README.md (placed after Configuration/Hooks, before Development) — one short paragraph per variable: what it does + when you'd set it. Add a matching compact section to AGENTS.md, which already has a Config section.

A reviewer (the no-mistakes document step) caught one accuracy issue in the initial draft: lifecycle hooks run inside pool.Acquire() before TREEHOUSE_DIR is added to the subshell env, so hooks do not see TREEHOUSE_DIR. The README wording was corrected to scope the variable to scripts inside the subshell and to explicitly note that hooks don't see it.

Scope

Two files, docs-only:

• README.md — new Environment Variables section.
• AGENTS.md — matching Environment Variables section.

No code changes.

Risk

None. Docs-only; zero build risk.

Verification

Validated through the no-mistakes pipeline: intent, rebase, review, test, document, and lint all passed with zero findings. The document step's accuracy fix (hook visibility of TREEHOUSE_DIR) is included in this PR and verified against the code.

AI disclosure

Human-reviewed. The documentation was drafted autonomously and reviewed by a human operator before opening. The accuracy correction regarding hook visibility of TREEHOUSE_DIR was produced by the no-mistakes document review step and verified against the code path in cmd/get.go and internal/hooks.