Make AGENTS.md the single agent guide and import it from CLAUDE.md

[erseco]

Stacked on #142. Last of the stack.

One agent guide

CLAUDE.md is now only @AGENTS.md. AGENTS.md is the single guide for every agent. Before this change the two files overlapped and contradicted each other.

CLAUDE.md described things that no longer match the code:

• .elp files and an "ElpParser";
• an editor git submodule on release/3.1-…, with a make update-submodule target;
• classes that no longer exist.

Its useful content moved into AGENTS.md, corrected, which now covers:

• How the plugin works: the upload → validate → extract → proxy flow, and components grouped by area (ingestion, reprocessing/CLI, content proxy and hash aliases, frontend, admin/styles, embedded editor).
• REST routes with their permissions, plus the post meta and options.
• The real editor build: a gitignored exelearning/ clone pinned by .editor-version, and build-editor-if-needed.
• The local environment: ports and credentials. The tests' uploads directory persists between runs, so fixtures are cleaned in tear_down() (see Clean up test extraction folders in tear_down #136).
• Translations through make translations (vendored WP-CLI), never the container's wp.
• Workflow rules: external values reach run: through env:, and persist-credentials: false is set where third-party packages are installed (see Harden the release workflows against injected values #139). The rule to keep actions on tags is kept.
• Boundary rule: don't drop allow-same-origin from package iframes piecemeal; the opaque-origin viewer lands as a whole in Opaque-origin published viewer and opt-in opaque editor preview #56.
• Block API: the stale ban on version 3, which the block has used since Move the block to Block API version 3, and two fixes from the #88 review #89, is dropped.

Meta keys are listed with their full _exelearning_ names. The branch rule now allows stacked PRs to target the previous branch. The exelearning.php comment about the file service now says .elpx.

The README's "Working with AI coding agents" section now describes the new arrangement.

make architecture-check passes. The ADR-68-01 references to CLAUDE.md "Data Storage" are pinned to a commit and left as accepted history.

[github-actions]

Test in WordPress Playground

Test the plugin with the code from this branch:

ℹ️ The eXeLearning editor is fetched from the shared release and unpacked into the plugin when the playground boots, so the first load may take a few extra seconds. ELP upload, shortcode, Gutenberg block and preview work normally.