chore(repo): add version-controlled rulesets for main, dev, and release tags

.github/rulesets/README.md

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+# Repository Rulesets
+
+These JSON files are version-controlled copies of the GitHub repository rulesets that protect this repo. They cannot be
+applied while the repository is private on a free-tier account — GitHub limits rulesets to public repos or paid (GitHub
+Pro / Team / Enterprise) tiers.
+
+The repo ships **PRIVATE** through v0.1.0 by design (see the master plan). Rulesets must be applied immediately after
+the visibility flip, before any post-flip pushes.
+
+## Files
+
+| File                | Target            | Effect                                                                                                                                                                                                                                |
+| ------------------- | ----------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
+| `protect-main.json` | `refs/heads/main` | Block direct creation/deletion/force-push; require PR with CODEOWNERS review, signed commits, linear history, and three required status checks: `markdownlint`, `shellcheck`, `guard-docs / check-forbidden-docs`. Squash-only merge. |
+| `protect-dev.json`  | `refs/heads/dev`  | Block deletion/force-push; require signed commits. Light-touch by design — `dev` is the integration branch.                                                                                                                           |
+| `protect-tags.json` | `refs/tags/v*`    | Block deletion, force-push (re-tag), and updates of release tags. Tags are immutable historical anchors that the site's `install.json` pins to.                                                                                       |
+
+## Apply (after visibility flip)
+
+```bash
+gh api repos/brettdavies/agentnative-skill/rulesets -X POST \
+  --input .github/rulesets/protect-main.json
+
+gh api repos/brettdavies/agentnative-skill/rulesets -X POST \
+  --input .github/rulesets/protect-dev.json
+
+gh api repos/brettdavies/agentnative-skill/rulesets -X POST \
+  --input .github/rulesets/protect-tags.json
+```
+
+## Verify
+
+```bash
+gh api repos/brettdavies/agentnative-skill/rulesets --jq '.[].name'
+# expected: Protect main, Protect dev, Protect release tags
+
+# negative test — force-push to main must be refused
+git checkout main
+git commit --allow-empty -m "test: should be rejected"
+git push origin main      # expected: refused by ruleset
+git reset --hard HEAD~1   # back out the local test commit
+
+# negative test — re-tagging an existing release tag must be refused
+git tag -f v0.1.0
+git push --force origin v0.1.0   # expected: refused by ruleset
+```
+
+## Bypass
+
+`actor_id: 5` (`RepositoryRole`) corresponds to repo admins, who retain bypass for emergency hotfixes and the visibility
+flip itself.

.github/rulesets/protect-dev.json

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+{
+  "name": "Protect dev",
+  "target": "branch",
+  "enforcement": "active",
+  "conditions": {
+    "ref_name": {
+      "exclude": [],
+      "include": [
+        "refs/heads/dev"
+      ]
+    }
+  },
+  "rules": [
+    {
+      "type": "deletion"
+    },
+    {
+      "type": "non_fast_forward"
+    },
+    {
+      "type": "required_signatures"
+    }
+  ],
+  "bypass_actors": [
+    {
+      "actor_id": 5,
+      "actor_type": "RepositoryRole",
+      "bypass_mode": "always"
+    }
+  ]
+}

.github/rulesets/protect-main.json

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+{
+  "name": "Protect main",
+  "target": "branch",
+  "enforcement": "active",
+  "conditions": {
+    "ref_name": {
+      "exclude": [],
+      "include": [
+        "refs/heads/main"
+      ]
+    }
+  },
+  "rules": [
+    {
+      "type": "creation"
+    },
+    {
+      "type": "deletion"
+    },
+    {
+      "type": "non_fast_forward"
+    },
+    {
+      "type": "pull_request",
+      "parameters": {
+        "required_approving_review_count": 0,
+        "dismiss_stale_reviews_on_push": false,
+        "required_reviewers": [],
+        "require_code_owner_review": true,
+        "require_last_push_approval": false,
+        "required_review_thread_resolution": false,
+        "allowed_merge_methods": [
+          "squash"
+        ]
+      }
+    },
+    {
+      "type": "required_signatures"
+    },
+    {
+      "type": "required_status_checks",
+      "parameters": {
+        "strict_required_status_checks_policy": true,
+        "do_not_enforce_on_create": false,
+        "required_status_checks": [
+          {
+            "context": "markdownlint"
+          },
+          {
+            "context": "shellcheck"
+          },
+          {
+            "context": "guard-docs / check-forbidden-docs"
+          }
+        ]
+      }
+    },
+    {
+      "type": "required_linear_history"
+    }
+  ],
+  "bypass_actors": [
+    {
+      "actor_id": 5,
+      "actor_type": "RepositoryRole",
+      "bypass_mode": "always"
+    }
+  ]
+}

.github/rulesets/protect-tags.json

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+{
+  "name": "Protect release tags",
+  "target": "tag",
+  "enforcement": "active",
+  "conditions": {
+    "ref_name": {
+      "exclude": [],
+      "include": [
+        "refs/tags/v*"
+      ]
+    }
+  },
+  "rules": [
+    {
+      "type": "deletion"
+    },
+    {
+      "type": "non_fast_forward"
+    },
+    {
+      "type": "update"
+    }
+  ],
+  "bypass_actors": [
+    {
+      "actor_id": 5,
+      "actor_type": "RepositoryRole",
+      "bypass_mode": "always"
+    }
+  ]
+}