fix: handle missing config file without panicking

[rnubel]

Summary

• Fixes a regression from 16bf1ea where LoadConfig would fatal-error when .pgmgr.json doesn't exist, breaking users relying solely on env-var configuration
• populateFromFile now silently ignores os.ErrNotExist when the config file path was not explicitly set by the user (i.e. uses the CLI default .pgmgr.json)
• All other errors (malformed JSON, permission denied, explicitly-provided missing paths) remain fatal

How It Works

argumentContext gains an IsSet(string) bool method (already implemented by cli.Context). LoadConfig passes ctx.IsSet("config-file") to populateFromFile, which uses it to distinguish:

• Default path not found => silently skip (env vars / CLI flags take over)
• Explicit --config-file or PGMGR_CONFIG_FILE not found => return error

[Copilot]

Pull request overview

Fixes a regression where LoadConfig errors when the default .pgmgr.json config file is missing, restoring support for env-var/flag-only configuration.

Changes:

• Extend the argumentContext interface with IsSet(string) bool and use it in LoadConfig.
• Update populateFromFile to ignore os.ErrNotExist only when the config path was not explicitly provided.
• Add tests covering missing default config behavior vs explicitly-provided missing paths (flag/env var).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
pgmgr/config.go	Passes IsSet("config-file") into populateFromFile to distinguish default vs explicit config paths and conditionally ignore missing default file errors.
pgmgr/config_test.go	Adds IsSet support to TestContext and introduces tests validating default-missing vs explicit-missing config file behavior.

Comments suppressed due to low confidence (1)

pgmgr/config.go:93

• json.Unmarshal(contents, &config) unmarshals into a **Config (pointer-to-pointer) because config is already a *Config receiver. This will update only the local pointer variable and will not populate the caller’s Config instance, so config-file settings won’t actually be applied. Unmarshal into the receiver itself (pass config, not &config).

		return err
	}
	if err := json.Unmarshal(contents, &config); err != nil {
		return err

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