Parent: #6853
Related implementation: #6855, #6944, #6945, #6946, #6947, #6948, #6949

Goal

Update extension framework documentation for merged SDK helper work and provide a migration guide for extension authors.

Why this matters

PR #6856 introduced new extension authoring surfaces; docs must be complete before broader rollout so external extension builders can adopt safely.

In scope

• D1: API reference updates for NewExtensionRootCommand, command scaffolding helpers, MCP server builder, ToolArgs, MCP result helpers, BaseServiceTargetProvider, and MCP security policy.
• D2: Migration cookbook from pre-feat: Extension SDK helpers — command scaffolding, MCP utilities, and security policy #6856 patterns to new helpers (before/after examples).
• D3: End-to-end sample flow (root setup + lifecycle commands + MCP tool registration).
• D4: Changelog/release notes updates pointing to migration docs.

Out of scope

• Net-new SDK feature design or implementation.
• Breaking behavioral changes to existing extension runtime.

Deliverables

• Updated extension framework docs pages with new API sections.
• Migration cookbook with concrete before/after snippets.
• One validated end-to-end sample flow.
• Release-note/changelog references.

Verification checklist

• Each helper area has at least one example.
• Docs navigation/index links resolve correctly.
• API names/options validated against current code.
• At least one extension maintainer can follow migration steps end-to-end.

Definition of done

• All deliverables complete and reviewed.
• No known documentation gaps for APIs shipped in feat: Extension SDK helpers — command scaffolding, MCP utilities, and security policy #6856.