refactor: SSOT for paired room prompts, remove 4 duplicate files

All reviewers (claude, codex-review, codex-review-failover) now use
claude-paired-room.md. All owners use owner-common-paired-room.md.

Removed: codex-paired-room.md, codex-review-paired-room.md,
codex-review-failover-paired-room.md, codex-review-platform.md

10 prompt files → 6.

src/agent-runner-environment.ts

@@ -269,29 +269,14 @@ function prepareCodexSessionEnvironment(args: {
                 'owner-common-paired-room.md',
               )
             : undefined,
-          args.isPairedRoom
-            ? readOptionalPromptFile(
-                args.projectRoot,
-                'codex-review-failover-paired-room.md',
-              )
-            : undefined,
           args.memoryBriefing,
         ]
       : [
           readPlatformPrompt('codex', args.projectRoot),
-          isReviewService(SERVICE_ID)
-            ? readOptionalPromptFile(
-                args.projectRoot,
-                'codex-review-platform.md',
-              )
-            : undefined,
           args.isPairedRoom
-            ? readPairedRoomPrompt('codex', args.projectRoot)
-            : undefined,
-          args.isPairedRoom && isReviewService(SERVICE_ID)
             ? readOptionalPromptFile(
                 args.projectRoot,
-                'codex-review-paired-room.md',
+                'owner-common-paired-room.md',
               )
             : undefined,
           args.memoryBriefing,

src/platform-prompts.ts

@@ -8,9 +8,11 @@ const PLATFORM_PROMPT_FILES: Record<AgentType, string> = {
   codex: 'codex-platform.md',
 };
 
+// SSOT: both agent types use the same paired room prompts.
+// Role-specific rules (owner vs reviewer) are selected by the caller.
 const PAIRED_ROOM_PROMPT_FILES: Record<AgentType, string> = {
   'claude-code': 'claude-paired-room.md',
-  codex: 'codex-paired-room.md',
+  codex: 'claude-paired-room.md',
 };
 
 export function getPlatformPromptsDir(projectRoot = process.cwd()): string {