feat(server): port /describe and /improve tools + real analytics

Phase 8 (extras) complete:
- describe.ts — full port of pr_description: type/title/description/diagram/
  per-file walkthrough table, PATCH /pulls to update description, optional
  labels. Verified live: PR #19 body updated with AI description (Type,
  Description bullets, mermaid diagram, file walkthrough table).
- improve.ts — full port of pr_code_suggestions summarize path:
  getPrMultiDiffs chunking, parallel LLM calls, score filtering, category
  table with unified diff snippets + persistent comment with history.
  Verified live: persistent update of existing '## PR Code Suggestions ✨'
  comment (updated_at 07:26:53) with new table.
- prompts.ts — verbatim pr_description_prompts.toml + code_suggestions prompts
- github.ts — getLineLink (SHA-256 diff anchor like pr_agent), getLabels,
  updateDescription (pulls.update); getPrMultiDiffs in diff.ts
- index.ts — real analytics: readLegacy pr-agent.*.log files (mtime-sorted,
  fixes pid-filename ordering bug), logReviewEvent writes legacy-format
  events, /api/metrics + /api/analytics now serve real data (427 events,
  per-command breakdown)
- cli.ts — --tool review|describe|improve
- tests 16/16, tsc clean

server/test/describe.test.ts

@@ -0,0 +1,39 @@
+// Tests for the PR description tool helpers (pure functions).
+
+import { describe, expect, test } from "bun:test";
+import {
+  deriveLabels,
+  sanitizeDiagram,
+  insertBrAfterXChars,
+} from "../src/describe";
+
+describe("describe helpers", () => {
+  test("deriveLabels from type list", () => {
+    expect(deriveLabels({ type: ["Bug fix", "Enhancement"] })).toEqual([
+      "Bug fix",
+      "Enhancement",
+    ]);
+    expect(deriveLabels({ type: "Bug fix, Tests" })).toEqual(["Bug fix", "Tests"]);
+    expect(deriveLabels({ labels: ["Bug fix"], type: ["Other"] })).toEqual(["Bug fix"]);
+    expect(deriveLabels({})).toEqual([]);
+  });
+
+  test("sanitizeDiagram strips fences and rejects non-diagrams", () => {
+    expect(sanitizeDiagram("```mermaid\nflowchart LR\n  A --> B\n```")).toBe(
+      "flowchart LR\n  A --> B",
+    );
+    expect(sanitizeDiagram("plain text without diagram")).toBe("");
+    expect(sanitizeDiagram("")).toBe("");
+    expect(sanitizeDiagram("graph TD\n  A --> B")).toBe("graph TD\n  A --> B");
+  });
+
+  test("insertBrAfterXChars wraps long text", () => {
+    const long = "a".repeat(80);
+    const wrapped = insertBrAfterXChars(long, 70);
+    expect(wrapped).toContain("<br>");
+    expect(wrapped.replace(/<br>/g, "")).toBe(long);
+    // html tags don't count toward the budget
+    const withTag = insertBrAfterXChars("<code>abcdef</code>", 70);
+    expect(withTag.length).toBe("<code>abcdef</code>".length);
+  });
+});