feat: Add January 2026 Threads API updates

[tirthpatell]

Summary

• Add author_username parameter to SearchOptions for filtering keyword/tag search results by author (January 20, 2026 changelog)
• Remove date-stamped comments from types.go and constants.go

January 2026 Changelog Assessment

Date	Item	Action
Jan 20	author_username search filter	Implemented
Jan 22	Web Intents tag/reply_control params	N/A - Web Intents, not API client
Jan 26	Webhook is_verified/profile_picture_url	N/A - no webhook types in library; existing Post struct already covers these fields
Jan 30	followers_count/follower_demographics for non-Instagram profiles	N/A - API-side policy change, no client restriction existed

Test plan

• go build ./... passes
• go vet ./... clean
• go test ./... -short passes
• Integration test with live API credentials

[greptile-apps]

Greptile Overview

Greptile Summary

Implements author_username search filter parameter added in January 20, 2026 Threads API changelog and removes date-stamped comments for cleaner code maintenance.

• Added AuthorUsername field to SearchOptions struct in types.go:330
• Implemented parameter handling in search.go:43-50 with automatic @ prefix stripping and validation
• Added test coverage for the new field in client_test.go:447-462
• Removed date-stamped comments from constants.go and types.go to improve maintainability

Confidence Score: 5/5

• This PR is safe to merge with minimal risk
• The changes are straightforward and well-implemented. The new author_username parameter follows existing patterns in the codebase for validation and parameter handling. The @ prefix stripping is a good UX improvement. Comment cleanup improves maintainability without affecting functionality. Test coverage is included.
• No files require special attention

Important Files Changed

Filename	Overview
search.go	Added author_username parameter support with proper validation and @ prefix stripping
types.go	Added AuthorUsername field to SearchOptions and removed date-stamped comments
client_test.go	Added basic test for AuthorUsername field in SearchOptions
constants.go	Removed date-stamped comments from field sets and error codes