chore(phase2): retarget all automation and contributor guidance from staged to main (#2122)

* chore(phase2): retarget all automation from staged to main

- publish.yml: trigger on main, publish only to marketplace
- check-pr-target.yml: invert — now blocks PRs targeting staged, welcomes main
- 10 PR validation workflows: branches [staged] → [main]
- external-plugin-command-router.yml: --base staged → main (3×), message text
- external-plugin-rereview-command.yml: --base staged → main (2×), message text
- external-plugin-rereview.yml: staged reference in review comment text
- external-plugin-intake.yml: ref: staged checkout → main
- external-plugin-pr-quality-gates.yml: ref: staged checkout → main
- external-plugin-quality-gates.yml: ref: staged checkout → main
- check-plugin-structure.yml: error messages updated for new branch model
- contributors.yml: ref and base target → main
- setup-labels.yml: targets-main label description updated
- cli-for-beginners-sync.md + .lock.yml: base-branch staged → main
- codeowner-update.md + .lock.yml: base-branch staged → main
- learning-hub-updater.md + .lock.yml: base-branch staged → main

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

* docs(phase2): update contributor guidance from staged to main

- CONTRIBUTING.md: branch from main, PR targets main; remove Phase 2 gate note
- AGENTS.md: PR target + external plugin PR automation references
- .github/pull_request_template.md: PR checklist targets main
- website/src/content/docs/learning-hub/agentic-workflows.md: PR target

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

* aw updates

---------

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

.github/workflows/external-plugin-rereview-command.yml

@@ -20,10 +20,10 @@ jobs:
       !github.event.issue.pull_request &&
       contains(github.event.comment.body, '/re-review-')
     steps:
-      - name: Checkout staged branch
+      - name: Checkout main branch
         uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4.3.1
         with:
-          ref: staged
+          ref: main
           fetch-depth: 0
 
       - name: Setup Node.js
@@ -226,7 +226,7 @@ jobs:
           git commit -m "Remove external plugin ${plugin_name} after six-month re-review"
           git push --force-with-lease origin "$branch"
 
-          pr_url=$(gh pr list --head "$branch" --base staged --json url --jq '.[0].url')
+          pr_url=$(gh pr list --head "$branch" --base main --json url --jq '.[0].url')
           if [ -z "$pr_url" ]; then
             pr_body=$(printf '%s\n' \
               '## Summary' \
@@ -235,7 +235,7 @@ jobs:
               '- regenerate marketplace outputs after the six-month re-review decision' \
               "- closes #${issue_number} review follow-up for this listing")
             pr_url=$(gh pr create \
-              --base staged \
+              --base main \
               --head "$branch" \
               --title "[external-plugin] Remove ${plugin_name} after re-review" \
               --body "$pr_body")
@@ -311,7 +311,7 @@ jobs:
                 issue_number: context.issue.number,
                 labels: ['re-review-due', 're-review-follow-up']
               });
-              body = `Opened the removal PR for **${pluginName}**: ${prUrl}. The issue remains approved and due for re-review until that removal lands in \`staged\`.`;
+              body = `Opened the removal PR for **${pluginName}**: ${prUrl}. The issue remains approved and due for re-review until that removal lands in \`main\`.`;
             }
 
             await github.rest.issues.createComment({