feat(plan): adapt planning workflow based on complexity of task (#20465)

Co-authored-by: Gal Zahavi <38544478+galz10@users.noreply.github.com>

docs/cli/plan-mode.md

@@ -80,14 +80,29 @@ manually during a session.
 
 ### Planning Workflow
 
+Plan Mode uses an adaptive planning workflow where the research depth, plan
+structure, and consultation level are proportional to the task's complexity:
+
 1.  **Explore & Analyze:** Analyze requirements and use read-only tools to map
-    the codebase and validate assumptions. For complex tasks, identify at least
-    two viable implementation approaches.
-2.  **Consult:** Present a summary of the identified approaches via [`ask_user`]
-    to obtain a selection. For simple or canonical tasks, this step may be
-    skipped.
-3.  **Draft:** Once an approach is selected, write a detailed implementation
-    plan to the plans directory.
+    affected modules and identify dependencies.
+2.  **Consult:** The depth of consultation is proportional to the task's
+    complexity:
+    - **Simple Tasks:** Proceed directly to drafting.
+    - **Standard Tasks:** Present a summary of viable approaches via
+      [`ask_user`] for selection.
+    - **Complex Tasks:** Present detailed trade-offs for at least two viable
+      approaches via [`ask_user`] and obtain approval before drafting.
+3.  **Draft:** Write a detailed implementation plan to the
+    [plans directory](#custom-plan-directory-and-policies). The plan's structure
+    adapts to the task:
+    - **Simple Tasks:** Focused on specific **Changes** and **Verification**
+      steps.
+    - **Standard Tasks:** Includes an **Objective**, **Key Files & Context**,
+      **Implementation Steps**, and **Verification & Testing**.
+    - **Complex Tasks:** Comprehensive plans including **Background &
+      Motivation**, **Scope & Impact**, **Proposed Solution**, **Alternatives
+      Considered**, a phased **Implementation Plan**, **Verification**, and
+      **Migration & Rollback** strategies.
 4.  **Review & Approval:** Use the [`exit_plan_mode`] tool to present the plan
     and formally request approval.
     - **Approve:** Exit Plan Mode and start implementation.