docs: make the README and integration guide agent-facing

[David Koleczek (DavidKoleczek)]

Why

• The snippet pointed at the repo root, whose lead is written for a human at a terminal (install, export a key, run a prompt). An agent asked to integrate this into a codebase had no destination written for it, so it improvised.
• Nothing stated that the engine is standalone, so an agent that searched the name found a large multi-repo ecosystem and reasonably concluded this was a sub-piece of it.
• No fit criteria, so an agent evaluating an unfamiliar dependency defaulted to suggesting one it already knew.

Changes

README

• The paste snippet now points at docs/INTEGRATION.md.
• What it is and Why are replaced by a definition plus a capability list. The per-turn subprocess mechanics were contract detail rather than orientation, and are already covered in the integration guide.
• Architecture at a glance states that the engine is standalone.

docs/INTEGRATION.md

• New Before you start section: the engine is standalone, no other microsoft/amplifier* repository substitutes for it, and it suits both agent loops and plain LLM calls via provider routing.

Accuracy

Capability claims were checked against the shipped bundle rather than carried over:

• Dropped recipes. tool-recipes is mounted, but no recipe content ships and only #subdirectory=modules/tool-recipes is pulled in, so the upstream examples do not resolve for a user.
• Dropped patching. tool-apply-patch defaults to the native engine, which is a host-side handler for the OpenAI Responses-API built-in tool. Provider-specific, and opaque to a README reader.
• Corrected provider selection to credentials-read-from-environment. Env-based provider detection is no longer performed on the run path; resolution is host config, then default_provider.
• Kept modes and skills, both of which ship real content and have test coverage.

Follow-ups (not in this PR)

• README.md quick start and docs/CONFIGURATION.md both document an auto-detection order that the run path no longer performs, and both omit github-copilot.
• bundle.md sets visibility.enabled: false for tool-skills while the adjacent comment claims visibility is enabled.