fix(agents): change Gemini model from 3.1-pro to 2.5-pro

- Update google/gemini-3.1-pro → google/gemini-2.5-pro (5 agents)
- Affected agents: ca-architecture-guard, ca-bug-hunter, ca-ref-reader, ca-spec-reader, ca-test-infra-improver
- Maintains large context capability for documentation and analysis tasks

.opencode/agents/ca-architecture-guard.md

@@ -8,7 +8,7 @@ description: >
 mode: subagent
 hidden: true
 temperature: 0.1
-model: google/gemini-3.1-pro
+model: google/gemini-2.5-pro
 color: warning
 permission:
   edit: deny

.opencode/agents/ca-bug-hunter.md

@@ -12,7 +12,7 @@ description: >
 mode: subagent
 hidden: true
 temperature: 0.1
-model: google/gemini-3.1-pro
+model: google/gemini-2.5-pro
 color: error
 permission:
   edit: deny

.opencode/agents/ca-ref-reader.md

@@ -7,7 +7,7 @@ description: >
 mode: subagent
 hidden: true
 temperature: 0.0
-model: google/gemini-3.1-pro
+model: google/gemini-2.5-pro
 color: info
 permission:
   edit: deny

.opencode/agents/ca-spec-reader.md

@@ -6,7 +6,7 @@ description: >
 mode: subagent
 hidden: true
 temperature: 0.0
-model: google/gemini-3.1-pro
+model: google/gemini-2.5-pro
 color: info
 permission:
   edit: deny

.opencode/agents/ca-test-infra-improver.md

@@ -12,7 +12,7 @@ description: >
 mode: subagent
 hidden: true
 temperature: 0.2
-model: google/gemini-3.1-pro
+model: google/gemini-2.5-pro
 color: "#2ECC71"
 permission:
   edit: deny