Support plugin-exported threshold defaults

[ColumbusLabs]

Goal

Let plugins ship default threshold values merged into getThreshold().

Difficulty

Medium

Area

Config

Suggested approach

• Extend plugin export shape with optional thresholds map.
• Merge after built-in defaults, before user config.

Acceptance criteria

• Example plugin demonstrates threshold export.
• User config still overrides plugin defaults.

Suggested test command

npm test -- tests/config/