Adds FirecrawlClient::developerSearch() to the PHP SDK. It calls POST /v2/search/developer and replaces the deprecated searchGithub(), which stops responding after 2026-11-03.

• DeveloperSearchOptions takes the 13 filters the API accepts: k, passages, types, repos, sources, language, topic, license, minStars, maxStars, archived, fork and skills.
• A blank query throws FirecrawlException before any request is sent.
• DeveloperSearchResponse exposes success, partial, results (id, url, title, passages, license) and repos / sources index status. license is an SPDX string or a disclosure array.
• The searchGithub() deprecation note now points at developerSearch().
• README example, CHANGELOG entry, unit tests with a mocked HTTP client, and a version bump from 1.16.5 to 1.16.6 (Version.php, which the publish workflow and the php-sdk@<version> origin tag both read).

Found by Wildfire, Firecrawl's integration testing: the developer-search check was failing against the PHP SDK.

Summary by cubic

Adds FirecrawlClient::developerSearch() to the PHP SDK, exposing the developer index at POST /v2/search/developer and replacing the deprecated searchGithub(), which stops responding after 2026-11-03.

• DeveloperSearchOptions covers all 13 API filters; a blank query throws FirecrawlException before any request is sent.
• DeveloperSearchResponse returns typed results, passages, and repo/source index status.
• Updates the searchGithub() deprecation note, README, CHANGELOG, and bumps the SDK to 1.16.6.

^{Written for commit c2ec453. Summary will update on new commits.}