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

[tarikermis]

Refs #1694. Complements #1858, which fixes the v1 prompt-turn page and explicitly left the v2 gap for a follow-up.

What & why

The v2 prompt-lifecycle pages walk a prompt turn through JSON examples for 9 session/update variants, but the schema's SessionUpdate union defines more:

• schema/v2/schema.json: 16 named variants + an other catch-all
• schema/v2/schema.unstable.json: 17 named variants (adds plan_removed) + an other catch-all

Variants such as available_commands_update, config_option_update, session_info_update, and terminal_update never appear on the page, and nothing marks the walkthrough as a subset — the same trap a client author reported in #1694 for v1 (spec-conformant traffic looking off-spec).

This PR updates both v2 copies, each matched to its own schema version:

• docs/protocol/v2/prompt-lifecycle.mdx (stable v2): 16-row table
• docs/protocol/v2/draft/prompt-lifecycle.mdx (draft/unstable): 17-row table, with plan_removed marked as draft-unstable; state_update also lists the unstable usage field on idle

Each page gets:

1. A <Note> at the start of "Agent Reports Output" clarifying the walkthrough covers the common prompt-turn variants only.
2. A new "Session Update Variants" section listing every schema-defined variant with what it reports, its key fields, and links to the topical pages (tool-calls, agent-plan, slash-commands, session-config-options, session-list) where each is documented in depth.
3. A short paragraph covering _-prefixed custom variants and the preserve-and-ignore rule for unrecognized variants, pointing at the existing Enum and Tagged-Union Variants section and the SessionUpdate schema reference — so no extensibility.mdx changes were needed (unlike v1 in docs: clarify the session/update variant set is not exhaustive #1858).

Verification

• Variant sets extracted programmatically from schema/v2/schema.json and schema/v2/schema.unstable.json; a script asserts each table matches its schema exactly (16/16 and 17/17, no extras).
• Key-fields column cross-checked against each variant's $defs properties/required lists (e.g. tool_call_update: required toolCallId + patch fields; usage_update: required used, size, optional cost; unstable IdleStateUpdate adds optional usage).
• Every intra-doc link anchor confirmed against the target headings in both the stable and draft trees.
• npx prettier --check passes on both files (repo CI format gate); npm run spellcheck (typos) passes repo-wide.

Review

The full diff was reviewed by kiro-cli (claude-opus-5), which independently verified the variant lists and field claims against agent-client-protocol-schema/src/v2 and the generated schema JSONs. Its findings (note placement splitting an example intro, Agent casing, plan_removed link text, missing unstable usage field, an over-long sentence) are all addressed in this revision.

Limitations

Docs-only change; I did not run mint dev to visually render the pages. The table/<Note> syntax mirrors existing usage on these pages and in #1858, and all anchors were verified statically.