bug: Cobra usage text and duplicate threshold messages in gaze quality output

[yvonnedevlinrh]

Problem

Two issues with gaze quality --min-contract-coverage output:

1. Usage text appears mid-output

The Cobra usage/help text appears in the middle of threshold violation messages, not at the end:

TestAnalyzeReturns_Direct_PureFunction: contract coverage 0% is below minimum 100%
Usage:
  gaze quality [packages...] [flags]
  ...
TestReturns_PureFunction: contract coverage 0% is below minimum 100%

This suggests either two separate code paths emit violation messages (one before and one after RunE returns its error),
or stderr buffering causes interleaving.

2. Duplicate violation lines

Many test-target pairs produce the same violation message twice:

TestAnalyzeP1Effects_Direct_ChannelSend: contract coverage 0% is below minimum 100%
TestAnalyzeP1Effects_Direct_ChannelSend: contract coverage 0% is below minimum 100%

Reproduction

gaze quality ./internal/analysis/ --min-contract-coverage=100

Expected Behavior

• Each violation message should appear exactly once
• No usage text in the output (threshold violations are runtime errors, not usage errors)

Investigation

1. Add SilenceUsage: true to the root Cobra command to fix the usage text issue
2. Trace the threshold check code path to find where violations are printed — look for two separate loops or a loop that runs over both per-package and merged results
3. Deduplicate violation messages