Make plugin guidance react to environment readiness

[sayan-oai]

Why

Generic plugin guidance is currently emitted only with initial context from host plugin state. An executor-selected plugin can become available later in the same turn, making its skills and tools usable without ever telling the model how plugin capabilities should be used.

What

• project every ready selected plugin package, including skill-only plugins
• carry plugin availability with the exact MCP runtime projection while preserving MCP manager reuse when servers and connectors are unchanged
• move generic plugin guidance from the static initial-context path into persisted World State
• recognize legacy and retained plugin fragments so resume and compaction do not duplicate guidance

Testing

• just test -p codex-mcp-extension
• just test -p codex-core plugins_instructions
• just test -p codex-core plugin_availability_change_reuses_the_mcp_manager
• just test -p codex-app-server --test all selected_capabilit