Issue #3636156: Move AGENTS.md to the repository root, make its checks runnable, and link the AI policy

Moves AGENTS.md and CLAUDE.md to the repository root. Agents started there now load them. Site projects no longer receive them through drupal/core.

Adds setup steps to §3 and §4. Adds §6: operator approval before posting, no self-RTBC, no credit changes, no pushing to another contributor's MR without agreement. Adds §7: read issues through the API, and the issue fork steps. Adds a link to the AI policy.

"Discouraged uses" and the disclosure line are unchanged, except for one section reference.

Tested with Claude Code 2.1.319 and Codex CLI 0.162.1, using a mock posting command. Without §6, agents posted in 8 of 8 runs. With §6, they posted in 0 of 8 runs. One message can approve several drafts.

This is a documentation-only change. ComposerIntegrationTest was run. Kernel, Functional, and FunctionalJavascript tests were not run for this change, because no code changed.

Limits are in the issue summary. Full test details are attached to the issue.

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