docs: add AI-agent migration guide for v8 to v9

Adds ai-docs/ai-migration.md — a compact (~4k token), machine-friendly
migration reference for AI coding agents like Claude Code and Cursor.

The human-oriented v8→v9 guide at
https://getstream.io/chat/docs/sdk/react-native/basics/upgrading-from-v8/
is ~40k tokens and eats most of an agent's context window before work
begins. This sibling doc strips rationale prose and keeps what agents
actually need: rename bullets, detection/verification greps, decision
rules for ambiguous cases, a machine-readable JSON rename block, and
pointers into the installed SDK source under
node_modules/stream-chat-react-native-core/src/.

Key agent-oriented features:

- §0 primer explicitly tells agents not to trust training data for v9
  symbols (which predate v9) and names the correct node_modules path,
  since 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 work by running a single rg pass
  and skipping sections whose patterns don't match.
- §3 forces the three structural migrations first (WithComponents, 5
  component renames, inverted audio semantics) because many leaf renames
  evaporate after the big moves.
- §6 covers behavior changes that aren't pure renames (messageContentOrder
  default swap, deleted-message visibility, swipe-to-reply boundary, etc.)
- §7 ships the full rename map as JSON for programmatic find/replace.
- §9 gives agents a gate: rg + tsc commands that must come back clean
  before declaring done.

Distribution: GitHub raw URL only — customers paste the link into their
agent. Not added to the npm package's files array.

Author: oliverlaz