fix(transcript): wrap reasoning in <thinking> tags in markdown export

The `_Thinking:_` open delimiter had no closing counterpart, so a
downstream parser couldn't tell where reasoning ends and the assistant
response begins in the exported markdown.

Switch the wrapper to `<thinking>...</thinking>`, which is well-formed
and matches the native output of reasoning models like DeepSeek, Qwen3,
and GLM. Live TUI rendering is intentionally untouched — that's #11439.

Closes #25308

Signed-off-by: 1fanwang <[email protected]>

Author: 1fanwang

packages/opencode/src/cli/cmd/tui/util/transcript.ts

@@ -88,7 +88,7 @@ export function formatPart(part: Part, options: TranscriptOptions): string {
 
   if (part.type === "reasoning") {
     if (options.thinking) {
-      return `_Thinking:_\n\n${part.text}\n\n`
+      return `<thinking>\n${part.text}\n</thinking>\n\n`
     }
     return ""
   }

packages/opencode/test/cli/tui/transcript.test.ts

@@ -149,7 +149,7 @@ describe("transcript", () => {
         time: { start: 1000 },
       }
       const result = formatPart(part, options)
-      expect(result).toBe("_Thinking:_\n\nLet me think...\n\n")
+      expect(result).toBe("<thinking>\nLet me think...\n</thinking>\n\n")
     })
 
     test("skips reasoning when thinking disabled", () => {