docs: agent-optimize README + add AGENTS.md/CLAUDE.md/llms.txt and IDE pointer files

[dawid-ai]

What this does

Optimizes this SDK's docs so AI coding agents and LLMs can discover the package, know when to use it vs its siblings, and use it correctly — without changing any code.

Changes

• README.md — adds a when to use this vs sibling packages disambiguation table, runnable examples verified against current source, real-world usage scenarios, an FAQ, and a ## For AI agents and LLMs section. Existing human-facing content is preserved and expanded.
• AGENTS.md (new) — agent-operational reference: build/test/lint commands, entry points, conventions, gotchas.
• CLAUDE.md (new) — thin pointer (@AGENTS.md) so Claude Code uses AGENTS.md as the single source of truth.
• llms.txt (new) — curated, resolvable link index for LLM crawlers.
• .github/copilot-instructions.md + .cursor/rules/cloudinary.mdc (new) — thin pointers to AGENTS.md so Copilot and Cursor share the same source of truth (no drift).

Notes

• Docs only — no source, dependency, or build changes.
• Every code example was verified against the current source before writing.
• This is a test PR for an agent-experience docs initiative across Cloudinary's repos; happy to adjust scope (e.g. split the README change out, or land only the additive files) based on your preference.

[dawid-ai]

cc @const-cloudinary — flagging for your review 🙏 (couldn't add you as a formal reviewer since this is a fork PR from a non-collaborator). This is the test PR for the agent-experience docs initiative we discussed — happy to adjust scope.