refactor(env): change array merge strategy from unique to replace

[pyramation]

Summary

Changes the deep merge array strategy from mergeArraysUnique (which concatenated and deduplicated arrays) to replaceArrays (which allows later configuration sources to completely override earlier ones).

Before: Arrays were merged and deduplicated

• ['uuid', 'postgis'] + ['postgis', 'hstore'] → ['uuid', 'postgis', 'hstore']

After: Later arrays completely replace earlier ones

• ['uuid', 'postgis'] + ['postgis', 'hstore'] → ['postgis', 'hstore']

This fixes the issue where arrays like api.metaSchemas with non-empty defaults could not be fully replaced by config/env/override values.

Review & Testing Checklist for Human

• Breaking change verification: This changes behavior for all array fields (db.extensions, jobs.*.supported, graphile.schema, api.exposedSchemas, api.metaSchemas, etc.). Verify no existing configurations rely on the old additive merge behavior.
• Export rename: mergeArraysUnique is now exported as replaceArrays. Check if any external code imports the old name from @pgpmjs/env.
• Test with real config: Try a real pgpm.json config that sets array values and verify overrides work as expected (later sources completely replace earlier ones).

Notes

• The user explicitly requested replace behavior after reviewing the tradeoffs
• Most array fields have empty defaults, so the practical impact is mainly on api.metaSchemas which had non-empty defaults

Link to Devin run: https://app.devin.ai/sessions/c09cd3880f9445d584dd937e037c2da4
Requested by: @pyramation

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