Fix OpenCode caveman installer

[dhruv-anand-aintech]

Summary

• add the missing OpenCode /caveman-compress command template so the installer no longer aborts mid-install
• convert copied cavecrew agent frontmatter from Claude-style tools arrays to OpenCode permission config during install
• add regression coverage for OpenCode-installed agent frontmatter

Verification

• npm test (51/51 passing)
• node --check bin/install.js && node --check src/plugins/opencode/plugin.js
• temp OpenCode install with node bin/install.js --only opencode --non-interactive --no-mcp-shrink --force exits 0 and generates permission-based cavecrew agents

[dhruv-anand-aintech]

Amended this branch with one review fix:

• skip OpenCode caveman-shrink MCP registration when no upstream MCP command is configured, because caveman-shrink is a proxy and exits when registered standalone
• added a regression test that runs the OpenCode installer with --with-mcp-shrink and asserts no broken caveman-shrink MCP entry is written

Validation:

• node --test tests/installer/opencode.test.mjs
• node --check bin/install.js
• git diff --check