fix(ci): pin wiki-sync second checkout to actions/checkout@v5 (#7759)

* chore(plugins): re-bundle document-mode and document Stage A path

Reverts the unbundling from b0cae69ffe. Stage 0 (gzip + throttle, shipped
in 84625be849) handles size; document-mode remains opt-in per context, so
cross-context conflict risk is bounded. Stage A (keyed plugin-persistence
API, issue #7749) is the documented future path for closing the LWW gap
on different-context concurrent edits — picked up when conflicts are
observed in practice.

Design sketch with multi-reviewed phasing lives in
docs/plans/2026-05-23-stage-a-keyed-plugin-persistence.md. Predecessor
plan's "Future work" section now links to it.

* test(plugins): cover document-mode bundled load and PLUGIN_USER_DATA LWW

Follow-ups from the multi-review of 199e816479's re-bundling decision:

- E2E smoke test asserts document-mode appears in plugin management so
  a typo in BUNDLED_PLUGIN_PATHS fails loudly.
- Spec exercises PLUGIN_USER_DATA conflict resolution end-to-end, which
  previously relied on analogy to REMINDER (same array+null branch) but
  was never directly asserted after the migration off 'virtual'.
- Stage A plan risks: stale-editor-view gap surfaced by the review;
  PluginHooks.PERSISTED_DATA_UPDATE already exists in the API but is
  never dispatched host-side — wiring it is the path to a fix.
- background.ts: comment marks the known gap at the registerHook site.

* docs(plugins): plan for wiring PERSISTED_DATA_CHANGED hook

Designs the host-side wiring for the currently-dead
PluginHooks.PERSISTED_DATA_CHANGED so plugins can react to remote-driven
changes to their persisted data. Multi-reviewed twice; v4 trims scope to
host-only (no plugin adoption in this design) and preserves the
multi-review insights as seeds for the follow-up doc-mode adoption
tracked at issue #7752.

Implementation lands in a separate PR.

* fix(ci): pin wiki-sync second checkout to actions/checkout@v5

v6 stores credentials in a $RUNNER_TEMP file referenced via
includeIf.gitdir matchers; the wiki push intermittently runs without
credentials on github-hosted runners (run 26335467138 failed with
"could not read Username for 'https://github.com'"). v5 uses the
path-independent extraheader, which is stable for this second
checkout pattern.

Leaves the primary Checkout Code on v6.0.2 so Dependabot keeps it
current.

Refs: actions/checkout#2321, #2359

.github/workflows/wiki-sync.yml

@@ -88,7 +88,11 @@ jobs:
           path: ${{ env.CODE_ROOT }}
 
       - name: Checkout Wiki
-        uses: actions/checkout@v6.0.2
+        # Pinned to v5: v6 changed credential persistence to a path-scoped
+        # includeIf matcher, which intermittently leaves the wiki push without
+        # credentials on github-hosted runners (run 26335467138). v5 uses the
+        # path-independent extraheader and is stable for this second checkout.
+        uses: actions/checkout@v5
         with:
           repository: ${{ github.repository }}.wiki
           path: ${{ env.WIKI_ROOT }}