feat(skills-next): add browser SDK references

[evanpurkhiser]

Direct LLM port of the existing browser SDK skill into the skills-next per-SDK reference layout under skills-next/references/sdks/browser/. This content has NOT been reviewed at all — it is a machine-generated port of the existing SDK skill, and every file should be treated as unverified.

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[evanpurkhiser]

Automated fixes pushed addressing the review feedback on this PR:

• session-replay.md: fixed the "Employee-only recordings" example. getReplay().flush() alone only uploads already-buffered data — it does not start recording or raise sampling, so unsampled employees got no replay. It now guards on getReplayId() and calls start() when no replay is active, with corrected prose.

Unreviewed LLM-port correction; please verify against the SDK.