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/cli-for-beginners-sync.md

@@ -13,7 +13,7 @@ safe-outputs:
   create-pull-request:
     labels: [automated-update, learning-hub, cli-for-beginners]
     title-prefix: "[bot] "
-    base-branch: staged
+    base-branch: main
 ---
 
 # CLI for Beginners Content Sync
@@ -116,7 +116,7 @@ Before opening the PR, write an updated `cli-for-beginners-sync-state.json` to `
 
 ## Step 6 — Open a pull request
 
-Create a pull request with your changes using the `create-pull-request` safe output. Use `staged` as the base branch for all work related to this workflow, and never branch from `main`. The PR body must include:
+Create a pull request with your changes using the `create-pull-request` safe output. Use `main` as the base branch for all work related to this workflow. The PR body must include:
 
 1. **What changed upstream** — a concise summary of the commits and file changes found in `github/copilot-cli-for-beginners`
 2. **What was updated locally** — list each mirrored Learning Hub file or asset you edited and what changed
@@ -132,7 +132,7 @@ If there is nothing to change after your analysis, do **not** open a PR. Instead
 - Only edit `website/astro.config.mjs` or `website/src/content/docs/learning-hub/index.md` when upstream course structure or navigation truly requires it
 - Preserve existing frontmatter fields; only update `lastUpdated` and `description` if genuinely warranted
 - Keep the course source-faithful; avoid summaries or interpretive rewrites
-- Use `staged` as the base branch for any branch or PR created by this workflow; never branch from `main`
+- Use `main` as the base branch for any branch or PR created by this workflow
 - Do not auto-merge; the PR is for human review
 - If you are uncertain whether an upstream change warrants a Learning Hub update, err on the side of creating the PR — a human reviewer can always decline
 - Always call either `create-pull-request` or `noop` at the end of your run so the workflow clearly signals its outcome