docs(zi-install): document machine setup interface

[ss-o]

z-shell/src PR 224 adds a versioned setup engine interface for guided clients. The canonical install skill still offered zunit for new installations and only described the human installer flow.

This update limits new setup choices to loader, annex, or install-only, keeps zunit compatibility-only, and documents the describe, plan, and apply artifact contract with exact plan approval, structured results, stable failures, and safe display handling.

Instruction impact review

1. Classification: Advisory skill guidance for Zi installation and setup clients. It does not add mandatory organization policy or enforcement.
2. Runtimes and contexts: All runtimes that select the existing skill-zi-install route for zi-installation or zi-setup tasks.
3. Canonical owner: .github/skills/zi-install/SKILL.md remains the correct canonical owner for the reusable install procedure. The source engine contract remains owned by z-shell/src.
4. Duplication or contradiction: No new instruction surface is created. The new text links the skill behavior to the source-owned contract and removes the prior contradiction that presented zunit as a new-install choice.
5. Manifest routes: No route change is required. skill-zi-install already targets the relevant tasks and all supported runtimes.
6. Mandatory delivery: This is an optional skill, not mandatory policy. Existing baseline and manifest delivery remain unchanged.
7. Generated output and limits: No generated instruction output changes. Public policy validation and the 32,768-byte limit pass in a clean repository export.

Validation

• python3 scripts/validate-agent-policy.py
• python3 -m unittest scripts/test_validate_agent_policy.py -v (92 passed)
• python3 -m unittest scripts/test_decision_records.py -v (17 passed)
• python3 -m unittest scripts/test_validate_zsh_standard_policy.py -v
• python3 scripts/validate-zsh-standard-policy.py
• python3 scripts/decision-records.py --check
• git diff --check

Refs z-shell/src#222 and z-shell/src#224.

[ss-o]

Superseded by #651, which uses the repository-required docs/ branch form.