[codex] bump pnpm pin to 11.6.0

[Junnyyy]

TL;DR

Pinned the repo to pnpm@11.6.0 and updated setup docs to match.

What changed?

• Updated packageManager in package.json from pnpm@11.5.0 to pnpm@11.6.0
• Updated README local setup prerequisites to name pnpm 11.6.0
• Added an AGENTS.md project learning that 11.6.0 is the intentional pin even when npm dist-tags are newer

How to test?

1. Run corepack pnpm --version and confirm 11.6.0
2. Run CI=true corepack pnpm install --frozen-lockfile
3. Run corepack pnpm build
4. Run corepack pnpm test
5. Run corepack pnpm typecheck
6. Run corepack pnpm lint
7. Run corepack pnpm latch report --flags packages/cli/examples/flags.json --src "packages/cli/examples/src/**/*.{ts,tsx}" packages/cli/examples/fixtures/flags.dev.json
8. Run corepack pnpm audit --audit-level moderate

Why make this change?

This aligns the repository package-manager pin with the pnpm version currently expected for the release branch, keeping Corepack-based installs and README setup guidance consistent.

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Note

Low Risk
Documentation and package-manager pin only; no application or CI logic changes in the diff.

Overview
Raises the workspace pnpm pin from 11.5.0 to 11.6.0 via root packageManager in package.json, so Corepack and local installs target the release-branch toolchain.

README local setup prerequisites now say pnpm 11.6.0 to match. AGENTS.md adds a note that 11.6.0 is the intentional pin even when npm’s latest-11 tag is newer.

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