chore: add a separate reject-policy flag for skill approvals (#14271)

## Summary
- add `skill_approval` to `RejectConfig` and the app-server v2
`AskForApproval::Reject` payload so skill-script prompts can be
configured independently from sandbox and rule-based prompts
- update Unix shell escalation to reject prompts based on the actual
decision source, keeping prefix rules tied to `rules`, unmatched command
fallbacks tied to `sandbox_approval`, and skill scripts tied to
`skill_approval`
- regenerate the affected protocol/config schemas and expand
unit/integration coverage for the new flag and skill approval behavior

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

@@ -29,6 +29,10 @@
                 },
                 "sandbox_approval": {
                   "type": "boolean"
+                },
+                "skill_approval": {
+                  "default": false,
+                  "type": "boolean"
                 }
               },
               "required": [