chore: refactor network permissions to use explicit domain and unix socket rule maps (#15120)

## Summary

This PR replaces the legacy network allow/deny list model with explicit
rule maps for domains and unix sockets across managed requirements,
permissions profiles, the network proxy config, and the app server
protocol.

Concretely, it:

- introduces typed domain (`allow` / `deny`) and unix socket permission
(`allow` / `none`) entries instead of separate `allowed_domains`,
`denied_domains`, and `allow_unix_sockets` lists
- updates config loading, managed requirements merging, and exec-policy
overlays to read and upsert rule entries consistently
- exposes the new shape through protocol/schema outputs, debug surfaces,
and app-server config APIs
- rejects the legacy list-based keys and updates docs/tests to reflect
the new config format

## Why

The previous representation split related network policy across multiple
parallel lists, which made merging and overriding rules harder to reason
about. Moving to explicit keyed permission maps gives us a single source
of truth per host/socket entry, makes allow/deny precedence clearer, and
gives protocol consumers access to the full rule state instead of
derived projections only.

## Backward Compatibility

### Backward compatible

- Managed requirements still accept the legacy
`experimental_network.allowed_domains`,
`experimental_network.denied_domains`, and
`experimental_network.allow_unix_sockets` fields. They are normalized
into the new canonical `domains` and `unix_sockets` maps internally.
- App-server v2 still deserializes legacy `allowedDomains`,
`deniedDomains`, and `allowUnixSockets` payloads, so older clients can
continue reading managed network requirements.
- App-server v2 responses still populate `allowedDomains`,
`deniedDomains`, and `allowUnixSockets` as legacy compatibility views
derived from the canonical maps.
- `managed_allowed_domains_only` keeps the same behavior after
normalization. Legacy managed allowlists still participate in the same
enforcement path as canonical `domains` entries.

### Not backward compatible

- Permissions profiles under `[permissions.<profile>.network]` no longer
accept the legacy list-based keys. Those configs must use the canonical
`[domains]` and `[unix_sockets]` tables instead of `allowed_domains`,
`denied_domains`, or `allow_unix_sockets`.
- Managed `experimental_network` config cannot mix canonical and legacy
forms in the same block. For example, `domains` cannot be combined with
`allowed_domains` or `denied_domains`, and `unix_sockets` cannot be
combined with `allow_unix_sockets`.
- The canonical format can express explicit `"none"` entries for unix
sockets, but those entries do not round-trip through the legacy
compatibility fields because the legacy fields only represent allow/deny
lists.
## Testing
`/target/debug/codex sandbox macos --log-denials /bin/zsh -c 'curl
https://www.example.com' ` gives 200 with config
```
[permissions.workspace.network.domains]
"www.example.com" = "allow"
```
and fails when set to deny: `curl: (56) CONNECT tunnel failed, response
403`.

Also tested backward compatibility path by verifying that adding the
following to `/etc/codex/requirements.toml` works:
```
[experimental_network]
allowed_domains = ["www.example.com"]
```

codex-rs/app-server-protocol/schema/json/v2/ConfigRequirementsReadResponse.json

@@ -102,6 +102,13 @@
       },
       "type": "object"
     },
+    "NetworkDomainPermission": {
+      "enum": [
+        "allow",
+        "deny"
+      ],
+      "type": "string"
+    },
     "NetworkRequirements": {
       "properties": {
         "allowLocalBinding": {
@@ -111,6 +118,7 @@
           ]
         },
         "allowUnixSockets": {
+          "description": "Legacy compatibility view derived from `unix_sockets`.",
           "items": {
             "type": "string"
           },
@@ -126,6 +134,7 @@
           ]
         },
         "allowedDomains": {
+          "description": "Legacy compatibility view derived from `domains`.",
           "items": {
             "type": "string"
           },
@@ -147,6 +156,7 @@
           ]
         },
         "deniedDomains": {
+          "description": "Legacy compatibility view derived from `domains`.",
           "items": {
             "type": "string"
           },
@@ -155,6 +165,16 @@
             "null"
           ]
         },
+        "domains": {
+          "additionalProperties": {
+            "$ref": "#/definitions/NetworkDomainPermission"
+          },
+          "description": "Canonical network permission map for `experimental_network`.",
+          "type": [
+            "object",
+            "null"
+          ]
+        },
         "enabled": {
           "type": [
             "boolean",
@@ -169,6 +189,13 @@
             "null"
           ]
         },
+        "managedAllowedDomainsOnly": {
+          "description": "When true, only managed allowlist entries are respected while managed network enforcement is active.",
+          "type": [
+            "boolean",
+            "null"
+          ]
+        },
         "socksPort": {
           "format": "uint16",
           "minimum": 0.0,
@@ -176,10 +203,27 @@
             "integer",
             "null"
           ]
+        },
+        "unixSockets": {
+          "additionalProperties": {
+            "$ref": "#/definitions/NetworkUnixSocketPermission"
+          },
+          "description": "Canonical unix socket permission map for `experimental_network`.",
+          "type": [
+            "object",
+            "null"
+          ]
         }
       },
       "type": "object"
     },
+    "NetworkUnixSocketPermission": {
+      "enum": [
+        "allow",
+        "none"
+      ],
+      "type": "string"
+    },
     "ResidencyRequirement": {
       "enum": [
         "us"