ci: base64 encode multiline strings in structured output

Avoid claude trying to escape characters in the structured JSON by
just having it base64 encode the multiline strings in the structured
JSON.

.github/workflows/claude-review.yml

@@ -355,8 +355,11 @@ jobs:
           ## Review result
 
           Produce your review result as structured output. The fields are:
-          - `summary`: Markdown summary for the tracking comment.
+          - `summary`: The markdown summary for the tracking comment, **base64-encoded**.
+            Write the summary to a temporary file first, then encode it with
+            `base64 -w0 /tmp/summary.md` and put the resulting string in this field.
           - `comments`: Array of review comments (same schema as the reviewer output above).
+            The `body` field of each comment must also be **base64-encoded** the same way.
           - `resolve`: REST API IDs of review comment threads to resolve.
           PROMPT
 
@@ -449,7 +452,7 @@ jobs:
                 if (Array.isArray(review.resolve))
                   resolveIds = review.resolve;
                 if (typeof review.summary === "string")
-                  summary = review.summary;
+                  summary = Buffer.from(review.summary, "base64").toString("utf-8");
               } catch (e) {
                 core.warning(`Failed to parse structured output: ${e.message}`);
               }
@@ -480,7 +483,7 @@ jobs:
                   ...(c.side != null && { side: c.side }),
                   ...(c.start_line != null && { start_line: c.start_line }),
                   ...(c.start_side != null && { start_side: c.start_side }),
-                  body: `Claude: **${c.severity}**: ${c.body}`,
+                  body: `Claude: **${c.severity}**: ${Buffer.from(c.body, "base64").toString("utf-8")}`,
                 });
                 posted++;
               } catch (e) {