Add Gemini CLI, Copilot, Cursor, and OpenCode installer targets #491

Problem

The installer core (#490) supports Bob only. Epic #482 also targets Gemini CLI, GitHub
Copilot, Cursor, and OpenCode.

Evidence

• Agent requirement docs: Document Gemini CLI install requirements #484 (Gemini CLI), Document GitHub Copilot install requirements #485 (Copilot), Document Cursor install requirements #486 (Cursor),
  Document OpenCode install requirements #487 (OpenCode) — each records install roots per scope and collision risks.
• The ADR (ADR: installer for non-marketplace agents #488) fixes the final target list; a target needing format translation is out.
• README.md:38-61 — Install section the core PR extends.

Expected

--target gemini|copilot|cursor|opencode (for each target the ADR kept) produces the tree
its doc specifies at each supported scope, with suite cases for install, re-run, update, and
conflict refusal per target, and README rows. just verify stays green.

Proposed approach

Add rows to the core's target table; no new code paths unless a doc proves one necessary.
Bump .claude-plugin/plugin.json (MINOR).

Part of #482
Blocked by #484
Blocked by #485
Blocked by #486
Blocked by #487
Blocked by #490