feat(plugin): expose app metadata

[thdxr]

Summary

• group optional host name, version, and channel under server app metadata
• expose normalized app metadata in V2 Effect and Promise plugin contexts
• centralize user-agent formatting in Core App.useragent()
• let CLI own injected OPENCODE_VERSION and OPENCODE_CHANNEL globals
• use app metadata across health, MCP, observability, models.dev, sessions, providers, report diagnostics, TUI, and simulation
• delete the shared Client context and util installation module

Testing

• full workspace typecheck
• Core MCP, provider, models, session, observability, web search, and report-skill tests
• Server and SDK Next tests
• CLI mini/server-connection tests
• Simulation frontend/backend tests