feat(skill): add sandbox config migration skill

.codex/skills/migrate-sandbox-config/SKILL.md

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+---
+name: migrate-sandbox-config
+description: Migrate the current Codex sandbox configuration in config.toml from legacy sandbox_mode and [sandbox_workspace_write] settings to the new permissions structure using default_permissions and [permissions.default]. Use when a user wants to convert their active sandbox config, repair a partial sandbox migration conservatively, or audit whether the current config can be migrated without changing behavior.
+---
+
+# Migrate Sandbox Config to Permissions
+
+## Overview
+
+Migrate only the top-level legacy sandbox settings in the target `config.toml`. Rewrite them to `default_permissions = "default"` plus populated `[permissions.default.filesystem]` and optional `[permissions.default.network]`. Prefer skipping with an explanation over changing behavior.
+
+Default to a dry run first. Show the exact proposed config changes and wait for explicit user confirmation before writing the file.
+
+## Workflow
+
+1. Read the target `config.toml` as text.
+2. Inspect only these keys before deciding what to do:
+   - top-level `sandbox_mode`
+   - top-level `[sandbox_workspace_write]`
+   - `default_permissions`
+   - `[permissions]`
+   - any `profiles.<name>.sandbox_mode`
+3. Decide whether migration is safe:
+   - If top-level `sandbox_mode` is missing, stop. Do not infer one.
+   - If active `default_permissions` and a non-empty `[permissions]` table already exist, stop.
+   - If `sandbox_mode = "danger-full-access"`, stop.
+   - If running on Windows and `sandbox_mode = "workspace-write"`, stop. Current behavior can depend on Windows sandbox support.
+4. Build `[permissions.default]` from the live legacy settings:
+   - `read-only` -> `[permissions.default.filesystem]` with `":root" = "read"`
+   - `workspace-write` -> `":root" = "read"` and `[permissions.default.filesystem.":project_roots"] "." = "write"`
+   - Copy each explicit `writable_root` as `"absolute/path" = "write"`
+   - If `network_access = true`, add `[permissions.default.network] enabled = true`
+   - Preserve `"/tmp" = "write"` only if it was explicitly writable already; do not invent extra temp paths
+   - Never migrate internal runtime-only writable roots such as Codex memories directories
+5. Preview the proposed migration before touching the file:
+   - Show the exact keys that will be added, removed, or replaced
+   - Call out behavior-changing fields explicitly, especially:
+     - network becoming enabled
+     - new writable roots outside the project root
+     - `"/tmp"` or `":tmpdir"` write access
+   - Mention any remaining profile-scoped legacy sandbox settings that will be left alone
+   - Wait for explicit user confirmation before editing the file
+6. Apply edits surgically after confirmation:
+   - Set `default_permissions = "default"`
+   - Remove top-level `sandbox_mode`
+   - Remove top-level `[sandbox_workspace_write]`
+   - Replace `permissions.default` before writing fresh leaves so stale keys do not survive
+   - Avoid creating empty `[permissions]` or `[permissions.default]` parent tables
+   - Preserve comments and unrelated settings when possible
+7. Verify:
+   - Re-read the file
+   - Confirm only populated `[permissions.default.*]` leaves were added
+   - Confirm `profiles.<name>.sandbox_mode` entries still exist and call them out in the summary
+
+## Partial Configs
+
+If `default_permissions` or `[permissions]` exists but there is still a live top-level legacy `sandbox_mode`, ignore stale `permissions.default` contents and rebuild from the legacy settings.
+
+If `default_permissions` or `[permissions]` exists and there is no top-level legacy `sandbox_mode`, stop and explain that the file is already partially migrated and Codex should not guess the intended behavior.
+
+## Output
+
+Summarize:
+- what was migrated
+- what was skipped
+- any remaining profile-scoped legacy sandbox settings

.codex/skills/migrate-sandbox-config/agents/openai.yaml

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+interface:
+  display_name: "Migrate Sandbox Config to Permissions"
+  short_description: "Migrate current sandbox config to permissions"
+  default_prompt: "Use $migrate-sandbox-config to migrate my current sandbox config in config.toml to the new permissions structure."