docs: add RFD 533 client migration guide

[lsaether]

Summary

• Add a client migration guide for the RFD #533-inspired attach/detach foundation from feat: add RFD 533-inspired attach/detach foundation #46 and attach response replay ordering from feat: add attach response replay ordering #47.
• Explain how clients should choose between legacy WebSocket replay and session/attach.result.history.
• Document historyPolicy, _meta.amux.replayOrder, appliedReplayOrder, pending permission replay/actionability, lifecycle expectations, and the Allow attach-aware clients to suppress legacy WS replay #48 legacy replay-suppression follow-up.
• Link the guide from README and the amux namespace design doc.

Test Plan

• git diff --check
• Validated all JSON code blocks in the new migration guide parse successfully.
• Validated touched local markdown links resolve.

Notes

Docs-only change; no Rust code touched.

[lsaether]

Clients should request server-shaped “newest turn first” attach history, not implement their own blind reverse-sort.

The client recipe is:

─ json
{
"method": "session/attach",
"params": {
"historyPolicy": "full",
"_meta": {
"amux": {
"replayOrder": "newest_turn_first"
}
}
}
}

Then:

1. Read transcript bootstrap from result.history.
2. Confirm result._meta.amux.appliedReplayOrder === "newest_turn_first".
3. Render that history as the bootstrap transcript.
4. Do not also render the legacy WebSocket replay.
5. Continue applying live WebSocket frames after attach.

And the key semantic: newest_turn_first means setup/context-ish frames stay first, completed turn groups are newest-first, butframes inside each turn stay chronological. So it is “freshest turns first,” not “reverse every event.”

[lsaether]

Closing this as stale under the new #70 direction. The attach/client migration details should be reworked as smaller AMUX-layer docs after the core-vs-AMUX boundary is refreshed.