[information-architecture] Migration guide omits permission-selection step for RWX/RWXP mappings

## Information Architecture Findings

### 1. Migration flow hides the access-level decision at the moment users remap workflows
**Area:** Picker placement / Progressive disclosure
**Component(s):** `docs/migration-guide.md`, `docs/workflows/claude-workflows.md`, `gh-agent-workflows/mention-in-pr/example.yml`, `gh-agent-workflows/mention-in-issue/example.yml`

**Problem:** The migration guide maps both legacy `rwx` and `rwxp` variants to the same target workflow file, but the migration checklist does not include any in-flow step to choose the corresponding `permissions:` level. Users doing a direct migration can complete the visible steps while missing the control that differentiates "no push" vs "push-capable" behavior.

**Evidence:**
- `docs/migration-guide.md:34-43` maps both `.../rwx` and `.../rwxp` to the same `gh-aw-*` workflow files.
- `docs/migration-guide.md:45-50` checklist includes no permission-selection step.
- `docs/workflows/claude-workflows.md:13` documents that push prevention depends on `permissions:` settings.
- `docs/workflows/claude-workflows.md:34-37` distinguishes `RWX` vs `RWXP` behavior.
- `gh-agent-workflows/mention-in-pr/example.yml:8-13` and `gh-agent-workflows/mention-in-issue/example.yml:6-11` show push-capable defaults (`contents: write`).

**Suggested improvement:** Add a permission-equivalence step immediately after **Workflow Mapping** in `docs/migration-guide.md` that explicitly maps legacy variant intent to required `permissions:` settings, with a short no-push (`contents: read`) and push-capable (`contents: write`) example.

## Suggested Actions
- [ ] Add a "Permission equivalence" subsection in `docs/migration-guide.md` directly after the mapping table.
- [ ] Add a migration checklist item requiring users to set `permissions:` explicitly based on whether they are migrating from no-push vs push-capable usage.
- [ ] Include one inline YAML snippet each for no-push and push-capable migrated workflows.

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