feat(agents): add centralized GPT apply_patch permission guard

Extract hardcoded GPT apply_patch permission logic into a reusable module
to ensure consistent behavior across all agents. This prevents GPT models
from using the unreliable apply_patch tool while allowing other models.

- Add gpt-apply-patch-guard.ts with GPT_APPLY_PATCH_GUIDANCE and getGptApplyPatchPermission
- Update Hephaestus agent to use centralized permission logic
- Update Sisyphus-Junior agent to use centralized permission logic
- Update all GPT prompt builders to reference shared guidance constant

🤖 Generated with assistance of OhMyOpenCode

src/agents/hephaestus/gpt.ts

@@ -1,5 +1,6 @@
 /** Generic GPT Hephaestus prompt - fallback for GPT models without a model-specific variant */
 
+import { GPT_APPLY_PATCH_GUIDANCE } from "../gpt-apply-patch-guard"
 import type {
   AvailableAgent,
   AvailableTool,
@@ -311,7 +312,7 @@ ${oracleSection}
 1. SEARCH existing codebase for similar patterns/styles
 2. Match naming, indentation, import styles, error handling conventions
 3. Default to ASCII. Add comments only for non-obvious blocks
-4. Use the \`edit\` and \`write\` tools for file changes. Do not use \`apply_patch\` on GPT models - it is unreliable here and can hang during verification.
+4. ${GPT_APPLY_PATCH_GUIDANCE}
 
 ### After Implementation (MANDATORY - DO NOT SKIP)