feat: Add JSON log format support

[abtreece]

Summary

• Add --log-format flag to switch between text and JSON log output
• Leverages existing logrus JSONFormatter for structured logging
• Makes log entries easier to parse by log indexing solutions (ELK, Splunk, etc.)

Changes

• Added SetFormat() function to pkg/log/log.go
• Added --log-format flag and log-format config option to cmd/confd/config.go
• Updated documentation in docs/command-line-flags.md and docs/configuration-guide.md

Usage

Command line:

confd -backend=env -log-format=json -onetime

Config file (confd.toml):

log-format = "json"

Example Output

Text format (default):

2026-01-07T01:30:00Z hostname confd[1234]: INFO Backend set to env

JSON format:

{"level":"info","msg":"Backend set to env","time":"2026-01-07T01:30:00Z"}

Test plan

• All existing tests pass
• Build succeeds
• -help shows new flag
• Manual testing with -log-format=json

Closes #177

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 9.09091% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 48.36%. Comparing base (ab7a2d5) to head (30ef66a).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/log/log.go	0.00%	8 Missing ⚠️
cmd/confd/config.go	33.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #327      +/-   ##
==========================================
- Coverage   48.55%   48.36%   -0.19%     
==========================================
  Files          19       19              
  Lines        1761     1772      +11     
==========================================
+ Hits          855      857       +2     
- Misses        825      834       +9     
  Partials       81       81              

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