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>

---------

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

.github/workflows/external-plugin-intake.yml

@@ -116,6 +116,7 @@ jobs:
                   vally_lint_status: 'infra_error',
                   smoke_status: 'infra_error',
                   version_match_status: 'infra_error',
+                  canvas_structure_status: 'infra_error',
                   failure_class: 'infra',
                   summary: 'Quality-gate workflow failed unexpectedly. Re-run intake to retry.',
                 };
@@ -127,6 +128,7 @@ jobs:
                   vally_lint_status: 'infra_error',
                   smoke_status: 'infra_error',
                   version_match_status: 'infra_error',
+                  canvas_structure_status: 'infra_error',
                   failure_class: 'infra',
                   summary: 'Quality-gate workflow did not return results. Re-run intake to retry.',
                 };