Fix plan approval gate and add "Create a plan:" prefix for first message

- Fix build warnings: add #[allow(dead_code)] to unused deserialization fields
- Fix plan approval gate bug: block file changes when no plan exists (not just
  when plan exists but is unapproved)
- Add "Create a plan: " prefix to first user message in plan mode
- Add prepare_plan_mode_input() helper function for testability
- Reset is_first_plan_message flag when entering plan mode via /plan command
- Add tests for approval gate (no plan + no changes, no plan + changes)
- Add tests for prepare_plan_mode_input (happy, negative, boundary cases)

crates/g3-cli/src/ui_writer_impl.rs

@@ -716,6 +716,7 @@ impl UiWriter for ConsoleUiWriter {
                 #[derive(serde::Deserialize)]
                 struct PlanCompact {
                     plan_id: String,
+                    #[allow(dead_code)]
                     revision: u32,
                     approved_revision: Option<u32>,
                     items: Vec<PlanItemCompact>,
@@ -731,6 +732,7 @@ impl UiWriter for ConsoleUiWriter {
                     #[serde(default)]
                     evidence: Vec<String>,
                     #[serde(default)]
+                    #[allow(dead_code)]
                     notes: Option<String>,
                 }
                 #[derive(serde::Deserialize)]