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-command-router.yml

@@ -23,10 +23,10 @@ jobs:
       !github.event.issue.pull_request &&
       (startsWith(github.event.comment.body, '/approve') || startsWith(github.event.comment.body, '/reject'))
     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
@@ -207,7 +207,7 @@ jobs:
           pr_url=""
           pr_number=""
           if git diff --quiet; then
-            pr_number=$(gh pr list --head "$branch" --base staged --json number --jq '.[0].number')
+            pr_number=$(gh pr list --head "$branch" --base main --json number --jq '.[0].number')
             if [ -n "$pr_number" ]; then
               pr_url=$(gh pr view "$pr_number" --json url --jq '.url')
             fi
@@ -227,7 +227,7 @@ jobs:
           git commit -m "${title_action} external plugin ${plugin_name}"
           git push --force-with-lease origin "$branch"
 
-          pr_number=$(gh pr list --head "$branch" --base staged --json number --jq '.[0].number')
+          pr_number=$(gh pr list --head "$branch" --base main --json number --jq '.[0].number')
           pr_body=$(cat <<EOF
           ## Summary
 
@@ -249,7 +249,7 @@ jobs:
             pr_url=$(gh pr view "$pr_number" --json url --jq '.url')
           else
             pr_url=$(gh pr create \
-              --base staged \
+              --base main \
               --head "$branch" \
               --title "[external-plugin] ${title_action} ${plugin_name}" \
               --body "$pr_body")
@@ -342,8 +342,8 @@ jobs:
               `- **Catalog action:** ${action}`,
               `- **Source repository:** \`${process.env.SOURCE_REPO}\``,
               prUrl
-                ? `- **PR against \`staged\`:** ${prUrl}`
-                : '- **PR against `staged`:** No new PR was needed because the approved listing is already present.'
+              ? `- **PR against \`main\`:** ${prUrl}`
+              : '- **PR against `main`:** No new PR was needed because the approved listing is already present.'
             ].join('\n');
 
             await syncIssueLabels(issueNumber, new Set(['external-plugin', 'approved']));
@@ -487,10 +487,10 @@ jobs:
       !github.event.issue.pull_request &&
       startsWith(github.event.comment.body, '/mark-ready-for-review')
     steps:
-      - name: Checkout staged branch
+      - name: Checkout main branch
         uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4.3.1
         with:
-          ref: staged
+          ref: main
 
       - name: Apply explicit ready-for-review override
         uses: actions/github-script@f28e40c7f34bde8b3046d885e986cb6290c5673b # v7.1.0
@@ -606,10 +606,10 @@ jobs:
       issue-state: ${{ steps.evaluate.outputs.issue-state }}
       issue-labels: ${{ steps.evaluate.outputs.issue-labels }}
     steps:
-      - name: Checkout staged branch
+      - name: Checkout main branch
         uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4.3.1
         with:
-          ref: staged
+          ref: main
 
       - name: Validate command and evaluate intake
         id: evaluate
@@ -716,10 +716,10 @@ jobs:
     needs: [rerun-intake-parse, rerun-intake-quality-gates]
     if: always() && needs.rerun-intake-parse.outputs.should-run == 'true'
     steps:
-      - name: Checkout staged branch
+      - name: Checkout main branch
         uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4.3.1
         with:
-          ref: staged
+          ref: main
 
       - name: Apply merged intake evaluation
         uses: actions/github-script@f28e40c7f34bde8b3046d885e986cb6290c5673b # v7.1.0