plan mode can transition mid-session to build mode on user request

Plan mode (read-only, plan file writes only) is not permanently sticky for a session. When the user explicitly shifts context (e.g., from "plan this" to "tell me everything you know" or to any task that requires file changes/shell), the operational mode transitions to build mode for the remainder of the session, unlocking full tool arsenal. Jul 1 2026 example: plan→build occurred when the user asked for a comprehensive knowledge synthesis. Recognize the trigger and stop restricting to plan-file writes.