chore(ci): replace mirror.yml with reusable wrapper (#16)

## Summary
Replaces this repo's full `mirror.yml` (~145 lines, drift-prone) with a
thin ~13-line wrapper that calls
`hyperpolymath/standards/.github/workflows/mirror-reusable.yml@e6b2884722350515934d443daf23442f2195796f`
(merged via standards#187).

Forge selection (GitLab, Bitbucket, Codeberg, SourceHut, Disroot, Gitea,
Radicle) remains gated by Actions `vars.<FORGE>_MIRROR_ENABLED` exactly
as before. `secrets: inherit` flows the per-forge SSH keys through
implicitly.

## Why
Estate audit: 289 `mirror.yml` deployments across the org, 75 unique
blob SHAs (76% drift). Drift is action-SHA pin churn, not feature
variance — the canonical 7-forge job set is identical across sampled
variants. Converging behind the reusable cuts ~94k LOC of estate
scaffold and means future changes to mirror logic propagate via one SHA
bump.

Part of estate-wide convergence campaign 2026-05-26 (standards#199 /
#187).

Author: hyperpolymath

.github/workflows/mirror.yml

@@ -1,72 +1,15 @@
 # SPDX-License-Identifier: MPL-2.0
-name: Mirror to GitLab and Bitbucket
+name: Mirror to Git Forges
 
 on:
   push:
-    branches: [main, master]
-    tags:
-      - 'v*'
+    branches: [main]
   workflow_dispatch:
 
-permissions: read-all
+permissions:
+  contents: read
 
 jobs:
-  mirror-gitlab:
-    runs-on: ubuntu-latest
-    permissions:
-      contents: read
-    if: ${{ vars.GITLAB_MIRROR_ENABLED == 'true' }}
-
-    steps:
-      - name: Checkout
-        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
-        with:
-          fetch-depth: 0
-
-      - name: Setup SSH
-        uses: webfactory/ssh-agent@e83874834305fe9a4a2997156cb26c5de65a8555 # v0.10.0
-        with:
-          ssh-private-key: ${{ secrets.GITLAB_SSH_KEY }}
-
-      - name: Add GitLab to known hosts
-        run: |
-          mkdir -p ~/.ssh
-          ssh-keyscan -t ed25519 gitlab.com >> ~/.ssh/known_hosts
-
-      - name: Push to GitLab
-        env:
-          REPO_NAME: ${{ github.event.repository.name }}
-        run: |
-          git remote add gitlab git@gitlab.com:hyperpolymath/${REPO_NAME}.git || true
-          git push gitlab HEAD:main --force || git push gitlab HEAD:master --force
-          git push gitlab --tags --force
-
-  mirror-bitbucket:
-    runs-on: ubuntu-latest
-    permissions:
-      contents: read
-    if: ${{ vars.BITBUCKET_MIRROR_ENABLED == 'true' }}
-
-    steps:
-      - name: Checkout
-        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
-        with:
-          fetch-depth: 0
-
-      - name: Setup SSH
-        uses: webfactory/ssh-agent@e83874834305fe9a4a2997156cb26c5de65a8555 # v0.10.0
-        with:
-          ssh-private-key: ${{ secrets.BITBUCKET_SSH_KEY }}
-
-      - name: Add Bitbucket to known hosts
-        run: |
-          mkdir -p ~/.ssh
-          ssh-keyscan -t ed25519 bitbucket.org >> ~/.ssh/known_hosts
-
-      - name: Push to Bitbucket
-        env:
-          REPO_NAME: ${{ github.event.repository.name }}
-        run: |
-          git remote add bitbucket git@bitbucket.org:hyperpolymath/${REPO_NAME}.git || true
-          git push bitbucket HEAD:main --force || git push bitbucket HEAD:master --force
-          git push bitbucket --tags --force
+  mirror:
+    uses: hyperpolymath/standards/.github/workflows/mirror-reusable.yml@e6b2884722350515934d443daf23442f2195796f
+    secrets: inherit