Add external plugin review board canvas for maintainers (#4174)

* feat: add external plugin review board canvas extension

Project-scoped Copilot canvas for maintainers to triage external plugin
submissions: kanban buckets, AI review, guided re-review via sub-session,
refresh, issue/comments view, and quick /approve or /reject.

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

* fix: harden review board per PR feedback

- Deny framing and hide stack traces from API error responses
- Track review queue ids, restore queue state on send failure, skip queued items
- Require reject reasons; treat posted decisions as pending until the workflow closes the issue
- Invalidate browser issue cache on refresh
- Clarify paid-services guidance

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

* fix: keep review decisions UI-only and record external outcomes

- Remove the agent-facing post_decision action to block prompt-injected approvals/rejections
- Record opposite terminal labels as external GitHub decisions instead of reusing the pending command
- State visibly that a reject reason is required

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

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

.gitignore

@@ -26,3 +26,6 @@ obj/
 bin/
 .impeccable
 .playwright-mcp
+
+# External plugin review board canvas state (local only)
+.github/extensions/external-plugin-review-board/state/