docs(unstable-v2): list all SessionUpdate variants on the prompt lifecycle page

[tarikermis]

Refs #1694 and complements #1858, which fixed the v1 page.

What changed

The v2 prompt-lifecycle pages show JSON examples for the common session/update variants, but they never say the walkthrough is incomplete. The schemas currently define 16 named variants plus other in stable v2, and 17 plus other in the unstable schema.

This updates both pages to match their schemas:

• docs/protocol/v2/prompt-lifecycle.mdx: all 16 stable variants.
• docs/protocol/v2/draft/prompt-lifecycle.mdx: all 17 unstable variants, including plan_removed and the unstable usage field on idle state updates.

Each page now has a note explaining that the walkthrough covers common prompt-turn updates, a complete variant table with key fields and links to the relevant topic pages, and a short explanation of custom _-prefixed variants and the preserve-and-ignore rule.

Verification

• Extracted both variant sets from the schema JSON and checked the tables against them: 16/16 stable and 17/17 unstable, with no extras.
• Cross-checked the key fields against the variant definitions.
• Verified every internal link and anchor in both documentation trees.
• Prettier passes on both files and the repository spellcheck passes.

I did not run mint dev; the table and <Note> syntax already exist on these pages and were checked statically.

Best Regards, Tarik