permissions: add built-in default profiles (#19900)

## Why

The migration away from `SandboxPolicy` needs new configs to start from
permissions profiles instead of deriving profiles from legacy sandbox
modes. Existing users can have empty `config.toml` files, and we should
not rewrite user-owned config files that may live in shared
repositories.

This PR introduces built-in profile names so an empty config can resolve
to a canonical `PermissionProfile`, while explicit named `[permissions]`
profiles still behave predictably.

## What changed

- Adds built-in `default_permissions` profile names:
  - `:read-only` maps to `PermissionProfile::read_only()`.
- `:workspace` maps to the workspace-write profile, including
project-root metadata carveouts.
- `:danger-no-sandbox` maps to `PermissionProfile::Disabled`, preserving
the distinction between no sandbox and a broad managed sandbox.
- Reserves the `:` prefix for built-in profiles so user-defined
`[permissions]` profiles cannot collide with future built-ins.
- Allows `default_permissions` to reference a built-in profile without
requiring a `[permissions]` table.
- Makes an otherwise empty config choose a built-in profile by
trust/platform context: trusted or untrusted project roots use
`:workspace` when the platform supports that sandbox, while roots
without a trust decision use `:read-only`.
- Keeps legacy `sandbox_mode` configs on the legacy path, and still
rejects user-defined `[permissions]` profiles that omit
`default_permissions` so we do not silently guess among custom profiles.
- Preserves compatibility behavior for implicit defaults: bare
`network.enabled = true` allows runtime network without starting the
managed proxy, explicit profile proxy policy still starts the proxy, and
implicit workspace/add-dir roots keep legacy metadata carveouts.

## Verification

- `cargo test -p codex-core builtin --lib`
- `cargo test -p codex-core profile_network_proxy_config`
- `cargo test -p codex-core
implicit_builtin_workspace_profile_preserves_add_dir_metadata_carveouts`
- `cargo test -p codex-core
permissions_profiles_network_enabled_allows_runtime_network_without_proxy`
- `cargo test -p codex-core
permissions_profiles_proxy_policy_starts_managed_network_proxy`

## Documentation

Public Codex config docs should mention these built-in names when the
`[permissions]` config format is ready to document as stable.









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codex-rs/config/src/config_toml.rs

@@ -114,7 +114,8 @@ pub struct ConfigToml {
     /// Sandbox configuration to apply if `sandbox` is `WorkspaceWrite`.
     pub sandbox_workspace_write: Option<SandboxWorkspaceWrite>,
 
-    /// Default named permissions profile to apply from the `[permissions]`
+    /// Default permissions profile to apply. Names starting with `:` refer to
+    /// built-in profiles; other names are resolved from the `[permissions]`
     /// table.
     pub default_permissions: Option<String>,