docs(all): add AGENTS.md and update development guide

[fefi95]

Closes #190
Closes #194

Verified (Node 25.2.1 from .nvmrc, fresh clone, cold cache)

Command	Result
npm install -g corepack && corepack enable	ok, yarn --version → 4.1.0
yarn install --immutable	ok, ~11 s
yarn compile / yarn lint / yarn test	exit 0; ~6 s / ~17 s / ~6 s
yarn compile && yarn lint && yarn test	exit 0, ~29 s
yarn check	exit 0, ~21 s
cd packages/core && yarn test	399 passing, ~1 s
yarn why tar	ok
yarn npm audit --all --recursive --severity high	exit 1 today (existing high findings, e.g. @isaacs/brace-expansion); exit 0 means clean
yarn dedupe tar --check	ok

Where the code disagreed with the issues

• Corepack on Node 25: .nvmrc pins 25.2.1, which no longer bundles Corepack, so a bare corepack enable fails. development.md adds npm install -g corepack for Node 25+.
• Commit scopes: there is no commitlint config in the repo. The scopes live only in the development.md table (plus per-package .releaserc.json rules), so AGENTS.md links to that table.
• usePlugin(): it is defined in packages/core/src/main.ts and calls config.addPlugin in config/Config.ts. AGENTS.md describes both.
• Lint: yarn lint prints react-native parse warnings but exits 0; AGENTS.md says to trust the exit code.
• Audit: added --severity high (per task) and noted the exit code.

AGENTS.md is the only agent file on purpose: it is the cross-tool convention, and #190's original title also mentioned CLAUDE.md.