docs: add AI-agent migration guide for v8 → v9 (#3557)

## Summary

Adds `ai-docs/ai-migration.md` — a compact, machine-friendly v8 → v9
migration reference built for AI coding agents (Claude Code, Cursor,
Copilot, etc.).

The human-oriented v8 → v9 guide at
https://getstream.io/chat/docs/sdk/react-native/basics/upgrading-from-v8/
is ~40k tokens. Loading it wholesale eats most of an agent's context
window before any code work begins. This sibling doc is ~4k tokens (5×
smaller) — prose rationale stripped, rename tables/decision rules/source
pointers kept.

Customers point their agent at the raw GitHub URL. It is **not** shipped
in the npm package.

### Key agent-oriented features

- **§0 primer** tells agents not to trust training data for v9 symbols
(their cutoff predates v9) and names the correct `node_modules` path —
the installed package is `stream-chat-react-native-core`, not
`stream-chat-react-native` (which is the bare-RN wrapper).
- **§1 detection greps** let agents scope the work with a single `rg`
pass and skip sections whose patterns don't match the customer codebase.
- **§3 big-3 structural migrations** enforce ordering: `WithComponents`,
5 component renames, inverted audio semantics — done first because many
leaf renames evaporate after the big moves.
- **§6 behavior changes** captures runtime-semantic shifts that aren't
pure renames (`messageContentOrder` default swap,
`deletedMessagesVisibilityType` removal, swipe-to-reply boundary,
`MessageActionType.type` field, etc.).
- **§7 JSON rename block** is the full v8 → v9 symbol map in a single
parseable block for programmatic find/replace.
- **§9 verification workflow** gives agents a hard gate: `rg` + `tsc`
commands that must come back clean before the agent declares done.

### Stacked on #3556

This PR targets `refactor/v9-finish-message-menu-and-audio-hook` so the
guide can accurately claim `useAudioPlayer` is the v9 hook name and
`MessageMenu` is removed. GitHub will auto-retarget to `develop` when
#3556 merges.

## Test plan

- [x] Verified every v9 symbol in the guide resolves in `package/src/`
- [x] Verified every `node_modules/` path referenced exists under
`package/src/`
- [x] Verified every `rg` pattern is syntactically valid (no exit-2 from
ripgrep)
- [x] Confirmed all "removed" claims in the JSON block are genuinely
absent from v9 source (only i18n translation keys and stale JSDoc
comments mention them)
- [ ] Dogfood check: paste the raw URL into Claude Code on a fresh v8
example app and have it migrate

## Related docs updates

Human migration guide edits (correcting the MessageMenu and
AudioAttachment claims that this agent guide is also correcting) are a
separate PR in the `docs-content` repo.

Author: oliverlaz