feat(company): serialize employees timeframes to topcompanies_prs.json

[PrestaEdit]

Summary

• Company::toArray() now embeds the full employees map — login → [{ start_date, end_date }, …] — sourced from var/data/companies.json.
• Traces already loaded this data to attribute each merged PR to the company employing the author at merge time; it just was never propagated to the JSON consumed by the front. Publishing it lets the front render a proper members table (entry/departure per period, still-active flag, multiple periods per person) instead of relying on the ad-hoc contributors list, which is the derived set of PR authors — a different, noisier population.
• The toArray() docblock is widened to array<string, mixed> because the shape now nests employees[].time_frames[]. phpstan (level max) is green on src/DTO/Company.php, and callers only consume the array via json_encode.

Test plan

• vendor/bin/phpstan analyse src/DTO/Company.php --level=max → No errors.
• php bin/console traces:generate:topcompanies → topcompanies_prs.json regenerated successfully; PrestaShop entry now carries employees (69 entries) alongside the pre-existing contributors (85 entries).
• Sample serialization matches the input:
  json { "login": "0x346e3730", "time_frames": [{ "start_date": "2022-10-03", "end_date": "2023-06-30" }] }
• Downstream front (TopContributors) consumes the new field to render a per-employee table with periods, tenure, and Active/Left status.

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