Fix fresh-install readiness and OpenCode migration

[matteopasseri407]

Summary

• Preserve OpenCode 1.18 JSONC configuration while reconciling canonical instructions and MCP servers.
• Detect the current OpenCode user install path from interactive shells and systemd user services.
• Report MULTI installs as BASE, PARTIAL, or READY, with an optional --require-ready gate for automation.
• Install the shared visible Chrome launcher, local-only CDP binding, Linux desktop entry, and compatibility redirect.
• Prevent sandbox readiness checks from launching real host CLIs and writing runtime state.

Verification

• 756 passed, 32 skipped in the complete agent universal layer test suite.
• bash install.sh --check passed with all four supported CLIs detected.
• Ruff baseline gate passed with no regressions.
• ShellCheck and Bash syntax checks passed.
• Working-tree and staged leak scans passed.
• Live Ubuntu verification confirmed OpenCode Go authentication and five connected MCP servers, including Firecrawl and Playwright.
• Live Chrome verification confirmed one personal profile with CDP listening only on 127.0.0.1:9222.
• Commit signature is verified locally and by GitHub.

Cross-platform status

The PowerShell twin and Windows source-contract tests are included.
Physical Windows verification remains pending until the Windows maintainer host is available, so this change must not be described as fully ported in the private operational backlog yet.