refactor(skills): remove zi-install, now in agent-skills (#741)

[ss-o]

Summary

The last removal step for #741: zi-install now lives in z-shell/agent-skills (z-shell/agent-skills#9, v0.5.0), is approved from there (#759), and its only consumer, z-shell/src, has re-pinned to it (z-shell/src#233). This PR removes the copy here.

• Deletes .github/skills/zi-install/SKILL.md and its skill-zi-install instruction surface.
• knowledge/domains/zi/index.md: the install task now links the skill in z-shell/agent-skills.
• knowledge/repository-files.json: the inventory entry is removed, and knowledge/domains/zi/repository-resources.md drops the matching generated row.
• Unchanged: the zi-install record in approved-skills.json (it already names agent-skills); the z-shell/src vendored_skills entry; the zi-install name in org-routing test fixtures, which model an external skill and never read this file.

Refs #741.

Verification

• org-routing.py validate; verify-approved --source-root z-shell/agent-skills=<clone of main>; validate-agent-policy.py; validate-zsh-standard-policy.py; knowledge-delivery.py --check (56 consumers); knowledge-coverage.py (356 files, generated pages current): pass.
• Unit tests in automation/agents, knowledge, governance and ci: OK.
• git grep skills/zi-install outside decisions/ finds only the agent-skills path.
• Trunk: Prettier would pad the two edited tables. Compact tables are the house style on main, and repository-resources.md is generated, so they stay unpadded.

Agent handoff

• Status: Ready for review
• Next steps: advance the meta-workspace pins (agent-skills and this repository) and close out Publish user and developer agent skills from a dedicated z-shell/agent-skills repository #741.
• Related tracker/issues: Publish user and developer agent skills from a dedicated z-shell/agent-skills repository #741

[ss-o]

Fallback review under ADR-0026: Copilot request not registered on c9b5b13

Findings

No actionable findings.

Scope: The complete diff against main (502ec61): removal of .github/skills/zi-install/SKILL.md and its surface, the inventory entry and generated resource row, and the Zi domain index link.

Note

This is a maintainer-elected self-review, not an independent review or approval to merge.

Validation

• Passed: all 12 checks on this head, including Org Routing's verify-approved against the agent-skills checkout.
• Local: routing validate and verify-approved, agent policy, Zsh standard policy, knowledge delivery and coverage, and unit tests in automation/agents, knowledge, governance and ci.

Checklist verdicts and supporting evidence

• Consumers moved first: pass. z-shell/src#233 re-pinned the only vendored copy to agent-skills before this removal, and its Org Routing caller is pinned to 502ec61, whose approved record already names the agent-skills source.
• No dangling references: pass. Outside decisions/, git grep skills/zi-install returns only agent-skills paths; the remaining zi-install names are the approved record and the test fixtures that model an external skill.
• Generated content: pass. knowledge-coverage.py confirms the resource page matches its generator.