feat(cli): add comprehensive environment variable support

[abtreece]

Summary

Add comprehensive environment variable support for CLI configuration, enabling Docker/Kubernetes users to configure confd entirely via environment variables.

Changes

• Add env: tags to 28 CLI struct fields in cmd/confd/cli.go
• Add tests for env var parsing and precedence in cmd/confd/cli_test.go
• Update documentation in docs/command-line-flags.md with complete env var reference

New Environment Variables

Core Settings

Variable	Description
CONFD_CONFDIR	Configuration directory
CONFD_CONFIG_FILE	Configuration file path
CONFD_INTERVAL	Polling interval (seconds)
CONFD_LOG_LEVEL	Log level (debug, info, warn, error)
CONFD_LOG_FORMAT	Log format (text, json)
CONFD_PREFIX	Key prefix
CONFD_WATCH	Enable watch mode
CONFD_ONETIME	Run once and exit
CONFD_NOOP	Dry-run mode
CONFD_SYNC_ONLY	Sync without check/reload commands
CONFD_FAILURE_MODE	Error handling mode
CONFD_KEEP_STAGE_FILE	Keep staged files

Timeouts

Variable	Description
CONFD_BACKEND_TIMEOUT	Backend operation timeout
CONFD_CHECK_CMD_TIMEOUT	Check command timeout
CONFD_RELOAD_CMD_TIMEOUT	Reload command timeout
CONFD_DIAL_TIMEOUT	Connection timeout
CONFD_READ_TIMEOUT	Read timeout
CONFD_WRITE_TIMEOUT	Write timeout
CONFD_PREFLIGHT_TIMEOUT	Preflight check timeout
CONFD_SHUTDOWN_TIMEOUT	Graceful shutdown timeout

Retry & Watch

Variable	Description
CONFD_RETRY_MAX_ATTEMPTS	Max retry attempts
CONFD_RETRY_BASE_DELAY	Initial backoff delay
CONFD_RETRY_MAX_DELAY	Maximum backoff delay
CONFD_DEBOUNCE	Watch mode debounce
CONFD_BATCH_INTERVAL	Batch processing interval
CONFD_WATCH_ERROR_BACKOFF	Watch error backoff

Caching

Variable	Description
CONFD_TEMPLATE_CACHE	Enable template caching
CONFD_STAT_CACHE_TTL	Stat cache TTL

Test plan

• Added TestCLIEnvVars - tests 11 env vars are parsed correctly
• Added TestCLIEnvVarPrecedence - verifies CLI flags override env vars
• Full test suite passes with race detector

Example Usage

# Docker example
docker run -e CONFD_INTERVAL=60 \
           -e CONFD_LOG_LEVEL=debug \
           -e CONFD_WATCH=true \
           -e CONFD_BACKEND_TIMEOUT=45s \
           confd etcd --node http://etcd:2379

Fixes #513

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR adds comprehensive environment variable support to confd's CLI configuration, enabling users to configure the tool entirely through environment variables—particularly useful for containerized deployments in Docker and Kubernetes.

Changes:

• Added env: tags to 28 CLI struct fields for environment variable support
• Added comprehensive test coverage for environment variable parsing and CLI flag precedence
• Expanded documentation with complete environment variable reference organized by category

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
cmd/confd/cli.go	Added env: tags to 28 struct fields enabling environment variable configuration
cmd/confd/cli_test.go	Added test functions to verify environment variable parsing and CLI flag precedence
docs/command-line-flags.md	Reorganized and expanded documentation with categorized tables for all environment variables

---

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 62.30%. Comparing base (780c075) to head (363e587).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #516      +/-   ##
==========================================
- Coverage   62.36%   62.30%   -0.06%     
==========================================
  Files          48       48              
  Lines        5420     5420              
==========================================
- Hits         3380     3377       -3     
- Misses       1831     1834       +3     
  Partials      209      209              

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[abtreece]

Findings:

• Medium: cmd/confd/cli_test.go:665 and cmd/confd/cli_test.go:787 call
  os.Unsetenv("ACM_EXPORT_PRIVATE_KEY") without restoring the original value;
  this mutates the process environment for subsequent tests and can leak into
  local runs. Use t.Cleanup (snapshot + restore) to keep the test hermetic.
• Medium: cmd/confd/cli_test.go:690 and cmd/confd/cli_test.go:804 rely on the
  ambient environment being free of other CONFD_* vars; if a developer has any
  set (especially invalid durations), kong.Parse can fail or alter
  expectations. Consider clearing/restoring CONFD_* variables around these
  tests.

Question:

• Docs now say “Global configuration can also be set via environment variables
  with the CONFD_ prefix,” but flags like --srv-domain, --srv-record,
  --watchdog-interval, --check-config, --validate, and diff flags still lack
  env bindings. Is that intentional, or should the wording be narrowed (or env
  tags expanded)?

[abtreece]

Addressed the review feedback:

Test fixes:

• Added clearCONFDEnvVars(t) helper that saves original values and restores them via t.Cleanup
• Helper clears all CONFD_* and ACM_EXPORT_PRIVATE_KEY env vars to ensure hermetic tests
• Both TestCLIEnvVars and TestCLIEnvVarPrecedence now use this helper

Additional env var support:

• Added CONFD_SRV_DOMAIN for DNS SRV domain
• Added CONFD_SRV_RECORD for SRV record discovery
• Added CONFD_WATCHDOG_INTERVAL for systemd watchdog

Not adding env vars for:

• --check-config, --validate, --preflight - validation flags are one-time operations, not suitable for persistent env config
• --diff, --diff-context, --color - diff flags are debugging aids, typically passed at runtime
• --mock-data, --resource - validation-specific flags

Documentation updated to include new service discovery section.