add Summarize mode for updatable PR summary comments (#470)

* add Summarize mode for updatable PR summary comments

Introduces a Summarize mode that manages a single summary comment per PR,
updated in place on subsequent pushes. Mirrors the Plan/PlanEdit pattern:
API endpoint for existing-comment lookup at select_mode time, node ID
tracking on WorkflowRun, and SummaryUpdate guidance for edits.

Also fixes summary format instructions: Before/After uses inline <br/>
to avoid double line breaks, metadata line placed after key changes,
SHA-256 anchor instructions strengthened against fabrication.

Made-with: Cursor

* fix pre-existing lint error in checkout.ts

Made-with: Cursor

* fix dead restricted param in deepenForBeforeSha

GitAuthOptions dropped the restricted field in the ASKPASS refactor (#478)
but deepenForBeforeSha (#471) still passed it. Remove the field and the
now-unused shell param from DeepenForBeforeShaParams.

Made-with: Cursor

mcp/server.ts

@@ -85,6 +85,8 @@ export interface ToolState {
   // set by select_mode when Plan + issue_number and plan-comment API returns existing plan (for report_progress target_plan_comment)
   existingPlanCommentId?: number;
   previousPlanBody?: string;
+  // set by select_mode when Summarize mode and summary-comment API returns existing summary
+  existingSummaryCommentId?: number;
   output?: string;
   usageEntries: AgentUsage[];
 }