fix(freehire-search): emit the contract's YYYY-MM-DD date instead of the API's ISO timestamp (#621)

toResult passed posted_at through verbatim, so search and detail JSON
carried "date": "2026-07-06T00:00:00Z" while every other portal CLI
emits the date part only and /scrape stores it as
posted_date: "YYYY-MM-DD". The table and plain renderers already cut the
value to ten characters, so only JSON consumers - the /scrape path - saw
the raw timestamp.

Add contractDate(): the timestamp's date part, null for an absent or
non-date value. The two tests that pinned the raw timestamp now expect
the date and fail on master, plus three cases on the helper. SKILL.md
documents the shape.

.agents/skills/freehire-search/cli/tests/commands.test.ts

@@ -98,7 +98,7 @@ describe("runSearch (mocked fetch)", () => {
     expect(parsed.meta).toEqual({ count: 1, page: 1, total: 42 });
     expect(parsed.results).toHaveLength(1);
     expect(parsed.results[0].id).toBe("backend-engineer-acme-ab12cd34");
-    expect(parsed.results[0].date).toBe("2026-07-06T00:00:00Z");
+    expect(parsed.results[0].date).toBe("2026-07-06");
   });
 
   test("queries the agent endpoint asking for full descriptions", async () => {