feat: add check-config subcommand for offline config validation

[cxdy]

Adds latr check-config so CI and local checks can validate configuration with the same load/default/validate logic as the running daemon, with no Linode or Vault side effects.

Usage

# Classic single file or glob
latr check-config --config config.yaml
latr check-config --config 'configs/*.yaml'

# Multi-file: process settings + token files
latr check-config --process process.yaml --config team-a.yml --config team-b.yml

# Directory: `_process.yaml` + other `*.yml`/`*.yaml` token files
latr check-config --dir path/to/config-dir

[Copilot]

Pull request overview

Adds an offline latr check-config subcommand so CI/local checks can validate configuration using the same parse/default/validate code paths as runtime, while avoiding any Linode/Vault side effects. This introduces multi-file and directory-based config composition helpers with accompanying unit tests, and documents the new workflow in the README.

Changes:

• Add check-config CLI subcommand (new flags/modes + exit codes) and update top-level CLI usage handling.
• Introduce config composition/loading helpers for multi-file merges and _process.(yml|yaml) + token-file directory layouts, plus validation helpers (e.g., duplicate label detection).
• Add README documentation and unit tests covering the new config composition and check-config behaviors.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
README.md	Documents latr check-config usage patterns, exit codes, and validation semantics.
internal/config/compose.go	Adds multi-file/dir composition loaders and a Check path for offline validation.
internal/config/compose_test.go	Adds unit tests for directory layouts, merge order, vault-credential requirements, and duplicate-label validation.
cmd/latr/main.go	Adds subcommand dispatch (check-config, version, help) while keeping legacy daemon flags working.
cmd/latr/check_config.go	Implements latr check-config flag parsing, mode selection, and exit-code behavior.

Comments suppressed due to low confidence (2)

internal/config/compose.go:122

• This error says the directory is missing _process.yaml, but LoadDir also accepts _process.yml. Consider mentioning both so users don’t hunt for the wrong filename.

		return nil, fmt.Errorf(
			"directory %s: missing process config %s",
			dir,
			ProcessConfigFileName,
		)

internal/config/compose.go:129

• This message refers to only _process.yaml, but _process.yml is also treated as the process config file. Updating the wording will make the directory layout expectations clearer.

		return nil, fmt.Errorf(
			"directory %s: need at least one token config file (*.yml / *.yaml) besides %s",
			dir,
			ProcessConfigFileName,
		)

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