Allow unlimited PR diff comment sections

[firewalker06]

Summary

• remove the five-section limit in the PR diff composer and server request validation
• retain per-section line/comment validation and duplicate-range protection
• show an unbounded section count and cover six-section submission

Closes #229

Verification

• bundle exec ruby test/remote_server_test.rb
• bundle exec ruby test/remote_ui_asset_snapshot_test.rb
• bin/test (fails in existing ManagedAgentTest.assert_poll_stops_stale_unstructured_output: expected stopped process to preserve its SIGTERM exit status)

Research

GitHub's review-comment API models comments per diff range and documents pagination (up to 100 per page), not a five-comment submission cap. Tycho only sends these validated draft sections to an agent; it does not post them to GitHub directly.