feat(transports/betterstack): add new Better Stack transport

[theogravity]

Summary

New transports/betterstack package that ships logs to Better Stack's HTTP intake API.

What's New

• Better Stack Transport: Full implementation with:
  • Source token authentication via Authorization: Bearer <token> header
  • Configurable timestamp field (default: dt)
  • Automatic metadata flattening at root level
  • Customizable HTTP headers (auth/content-type replaced, user headers preserved)
  • Comprehensive unit tests and livetest support

Key Features

• Wraps transports/http for batched delivery with configurable BatchSize/BatchInterval
• Supports all log levels (trace, debug, info, warn, error, fatal, panic)
• Properly handles metadata fields as flat JSON properties
• URL validation enforces HTTPS unless AllowInsecureURL=true
• HTTP headers merge: auth/content-type replaced, custom headers preserved

Testing

go test ./transports/betterstack/ -v          # all tests pass
go test -tags=livetest ./transports/betterstack/ -v  # live API (requires BETTERSTACK_SOURCE_TOKEN)

Documentation

• Added docs/src/transports/betterstack.md
• Updated sidebar in docs/.vitepress/config.ts (Cloud section)
• Added to transport list partial
• Updated llms.txt, llms-full.txt

Changelog

Changeset added: .changeset/tiny-wren.md

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resolves: #88