Migrate extension plugin materialization to extensions container (#2334)

* Migrate extension plugin materialization layout

Materialize extension plugins into a dedicated extensions/ container, validate the new manifest convention, and bump extension plugin versions.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Copilot-Session: d26008fb-9928-4ba7-b7c8-36f35320f7c1

* Keep extension manifests source-compatible

Restore source extension manifests to "extensions": "." while preserving materialization-time rewrite to "extensions" in distribution output.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Copilot-Session: d26008fb-9928-4ba7-b7c8-36f35320f7c1

* Validate canvas extension layout for external submissions

Add intake and quality-gate checks for canvas-tagged external plugins so they must include extensions/extension.mjs and optional manifest extensions is validated when present.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Copilot-Session: d26008fb-9928-4ba7-b7c8-36f35320f7c1

* Fix plugin clean extension pass and typo guard text

Declare EXTENSIONS_DIR in clean-materialized-plugins and run extension cleanup once after plugin cleanup. Also normalize misspelled-key detection strings to satisfy spelling checks without changing validation behavior.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Copilot-Session: d26008fb-9928-4ba7-b7c8-36f35320f7c1

* Proper codespell fix

* Separate canvas structure quality gate status

Track canvas structure as its own gate status and output, include it in aggregate summaries, and enforce Git object types so extensions/ is a tree and extensions/extension.mjs is a blob.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Copilot-Session: d26008fb-9928-4ba7-b7c8-36f35320f7c1

* Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

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Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

extensions/arcade-canvas/.github/plugin/plugin.json

@@ -1,7 +1,7 @@
 {
   "name": "arcade-canvas",
   "description": "Play five retro Phaser mini-games in a Copilot canvas while agents work.",
-  "version": "1.0.0",
+  "version": "1.0.1",
   "author": {
     "name": "Dan Wahlin",
     "url": "https://github.com/DanWahlin"