fix: make root marketplace catalog real files (not symlinks)

[jpelletier1]

• A human has tested these changes.

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Why

When the plugin-directory is configured with MARKETPLACE_SOURCE=github://openhands/extensions, the marketplace catalog does not render (empty Plugin Directory).

Root cause: the catalog was exposed through a double symlink chain, and raw.githubusercontent.com does not dereference git symlinks (it serves the literal symlink target text; for symlinked directory paths it returns 404):

• .claude-plugin (symlink) -> .plugin
• .plugin/marketplace.json (symlink) -> ../marketplaces/openhands-extensions.json

Verified empirically against main: GET .plugin/marketplace.json returns HTTP 200 but the body is the literal string ../marketplaces/openhands-extensions.json (not JSON), and GET .claude-plugin/marketplace.json returns 404 (symlinked directory not traversable). So consumers doing GET + json.loads() get a 404 or a parse error and an empty catalog. This is a server-side (GitHub raw) limitation the plugin-directory client cannot work around, so the catalog simply never arrives.

Summary

• Replaced the root vendor symlinks (.claude-plugin, .codex-plugin, .plugin/marketplace.json) with real directories + real marketplace.json files that each mirror the canonical marketplaces/openhands-extensions.json byte-for-byte (canonicalized JSON). Git mode changed from 120000 (symlink) to 100644 (regular file). The catalog is now reachable at every standard .claude-plugin/marketplace.json / .codex-plugin/marketplace.json / .plugin/marketplace.json path over the raw URL.
• Added a root-marketplace sync task to scripts/sync_extensions.py that keeps all three root copies in sync with the canonical file and reports drift in --check mode. Since CI runs python scripts/sync_extensions.py --check with no task filter, the new task is automatically enforced on every PR.
• Added tests in tests/test_sync_extensions.py (TestRootMarketplace) verifying the real-repo files are non-symlinks, match the canonical catalog, are valid JSON, and that the sync task detects/fixes both the original symlink chain and stale copies.
• Updated AGENTS.md with the invariant (root catalog copies are real files, never symlinks).

Issue Number

Closes #394

How to Test

1. pip install pyyaml pytest
2. python scripts/sync_extensions.py --check - exits 0 (only the pre-existing, non-blocking coverage warning for plugins/issue-duplicate-checker remains).
3. python -m pytest tests/test_sync_extensions.py -q - 40 passed.
4. Confirm the files are real (mode 100644, not 120000): git ls-files -s -- .claude-plugin .codex-plugin .plugin
5. (Optional, network) After merge to main, curl -fsSL https://raw.githubusercontent.com/OpenHands/extensions/main/.claude-plugin/marketplace.json returns the catalog JSON (HTTP 200) instead of 404. (Cannot verify pre-merge from this branch since the raw URL serves main.)

Video/Screenshots

N/A - file-system / raw-URL fix with no UI surface.

Notes

• The .claude-plugin/marketplace.json, .codex-plugin/marketplace.json, and .plugin/marketplace.json files are now generated copies; edit the canonical marketplaces/openhands-extensions.json and run python scripts/sync_extensions.py (no args) to refresh all three. Do not reintroduce symlinks at these paths.
• Per-skill/per-plugin .claude-plugin and .codex-plugin vendor symlinks within skills/ and plugins/ are unrelated and unchanged (managed by the existing symlinks sync task).
• JSON is canonicalized (sorted keys, 2-space indent) for stable, minimal diffs and reliable byte-for-byte equality checks.

This PR was created by an AI agent (OpenHands) on behalf of a user.

[jpelletier1]

@jpshackelford As someone passionate about Plugins, I wanted to see if you had a chance to review this PR. Specifically, the symlinks we use in the OpenHands/extensions repo makes it difficult to render the full plugin marketplace in the Plugin Directory -- this attempts to fix that by creating an actual marketplace.json in the respective folders and keeping them in sync.