Update extension framework docs

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-#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.
- [x] One validated end-to-end sample flow.
- [x] Release-note/changelog references.

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

## Definition of done
- [ ] All deliverables complete and reviewed.
- [x] No known documentation gaps for APIs shipped in #6856.