test: add packaged OpenCode smoke gate

[sven1103-agent]

Summary

• add an isolated OpenCode smoke test that runs against the packed plugin artifact and verifies the plugin registers its custom tools
• add an opt-in published-package smoke path so we can reproduce the current latest failure and validate release channels separately
• run the packed-artifact smoke test in both PR CI and publish CI before shipping

Testing

• OPENCODE_BIN="/Users/sven1103/.opencode/bin/opencode" npm run test:smoke
• OPENCODE_BIN="/Users/sven1103/.opencode/bin/opencode" OPENCODE_SMOKE_PUBLISHED_SPEC="@sven1103/opencode-worktree-workflow" npm run test:smoke:published
• OPENCODE_BIN="/Users/sven1103/.opencode/bin/opencode" OPENCODE_SMOKE_PUBLISHED_SPEC="@sven1103/[email protected]" npm run test:smoke:published

Context

• reproduces the current latest breakage (0.6.3 -> Plugin export is not a function)
• confirms the 1.0.0-alpha.3 channel and the local packed artifact load successfully