feat: introduce Permissions (#11633)

## Why
We currently carry multiple permission-related concepts directly on
`Config` for shell/unified-exec behavior (`approval_policy`,
`sandbox_policy`, `network`, `shell_environment_policy`,
`windows_sandbox_mode`).

Consolidating these into one in-memory struct makes permission handling
easier to reason about and sets up the next step: supporting named
permission profiles (`[permissions.PROFILE_NAME]`) without changing
behavior now.

This change is mostly mechanical: it updates existing callsites to go
through `config.permissions`, but it does not yet refactor those
callsites to take a single `Permissions` value in places where multiple
permission fields are still threaded separately.

This PR intentionally **does not** change the on-disk `config.toml`
format yet and keeps compatibility with legacy config keys.

## What Changed
- Introduced `Permissions` in `core/src/config/mod.rs`.
- Added `Config::permissions` and moved effective runtime permission
fields under it:
  - `approval_policy`
  - `sandbox_policy`
  - `network`
  - `shell_environment_policy`
  - `windows_sandbox_mode`
- Updated config loading/building so these effective values are still
derived from the same existing config inputs and constraints.
- Updated Windows sandbox helpers/resolution to read/write via
`permissions`.
- Threaded the new field through all permission consumers across core
runtime, app-server, CLI/exec, TUI, and sandbox summary code.
- Updated affected tests to reference `config.permissions.*`.
- Renamed the struct/field from
`EffectivePermissions`/`effective_permissions` to
`Permissions`/`permissions` and aligned variable naming accordingly.

## Verification
- `just fix -p codex-core -p codex-tui -p codex-cli -p codex-app-server
-p codex-exec -p codex-utils-sandbox-summary`
- `cargo build -p codex-core -p codex-tui -p codex-cli -p
codex-app-server -p codex-exec -p codex-utils-sandbox-summary`

codex-rs/core/tests/suite/client.rs

@@ -973,8 +973,8 @@ async fn user_turn_collaboration_mode_overrides_model_and_effort() -> anyhow::Re
                 text_elements: Vec::new(),
             }],
             cwd: config.cwd.clone(),
-            approval_policy: config.approval_policy.value(),
-            sandbox_policy: config.sandbox_policy.get().clone(),
+            approval_policy: config.permissions.approval_policy.value(),
+            sandbox_policy: config.permissions.sandbox_policy.get().clone(),
             model: session_configured.model.clone(),
             effort: Some(ReasoningEffort::Low),
             summary: config.model_reasoning_summary,