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/check-plugin-structure.yml

@@ -2,7 +2,7 @@ name: Check Plugin Structure
 
 on:
   pull_request:
-    branches: [staged]
+    branches: [main]
     paths:
       - "plugins/**"
 
@@ -93,8 +93,8 @@ jobs:
                   if (files.length > 0) {
                     errors.push(
                       `${pluginPath}/${subdir}/ contains ${files.length} file(s): ${files.join(', ')}. ` +
-                      `Plugin directories on staged should only contain .github/plugin/plugin.json and README.md. ` +
-                      `Agent, command, and skill files are materialized automatically during publish to main.`
+                      `Plugin directories on main should only contain .github/plugin/plugin.json and README.md. ` +
+                      `Agent, command, and skill files are materialized automatically during publish to marketplace.`
                     );
                   }
                 }
@@ -120,11 +120,11 @@ jobs:
               const body = [
                 '⚠️ **Materialized files or symlinks detected in plugin directories**',
                 '',
-                'Plugin directories on the `staged` branch should only contain:',
+                'Plugin directories on the `main` branch should only contain:',
                 '- `.github/plugin/plugin.json` (metadata)',
                 '- `README.md`',
                 '',
-                'Agent, command, and skill files are copied in automatically when publishing to `main`.',
+                'Agent, command, and skill files are copied in automatically when publishing to `marketplace`.',
                 '',
                 '**Issues found:**',
                 ...errors.map(e => `- ${e}`),
@@ -133,12 +133,12 @@ jobs:
                 '',
                 '### How to fix',
                 '',
-                'It looks like your branch may be based on `main` (which contains materialized files). Here are two options:',
+                'It looks like your branch may include materialized plugin files that should not be on `main`. Here are two options:',
                 '',
-                '**Option 1: Rebase onto `staged`** (recommended if you have few commits)',
+                '**Option 1: Rebase to drop materialized files** (recommended if you have few commits)',
                 '```bash',
-                `git fetch origin staged`,
-                `git rebase --onto origin/staged origin/main ${prBranch}`,
+                `git fetch origin main`,
+                `git rebase --onto origin/main origin/main ${prBranch}`,
                 `git push --force-with-lease`,
                 '```',
                 '',
@@ -179,7 +179,7 @@ jobs:
                 core.warning(body);
               }
 
-              core.setFailed('Plugin directories contain materialized files or symlinks that should not be on staged');
+              core.setFailed('Plugin directories contain materialized files or symlinks that should not be on main');
             } else {
               console.log('✅ All plugin directories are clean');
             }