* fix(memory): allow OMNIROUTE_STRICT_SYSTEM_PROVIDERS to extend the system-first provider list
PROVIDERS_SYSTEM_MUST_BE_FIRST (added in #6225 for #6135) gates both the
memory-injection placement fix and the #7293 hoistLeadingSystemMessage
translator fix, but was hardcoded to xiaomi-mimo/mimo only. Self-hosted
deployments routing other strict backends (e.g. a custom OpenAI-compatible
connection in front of a self-hosted Qwen3.5+/3.6 model, whose chat template
rejects any non-leading system message the same way) had no way to opt in
without forking and rebuilding the image.
Adds OMNIROUTE_STRICT_SYSTEM_PROVIDERS (comma-separated, case-insensitive
provider ids) to extend the built-in set at read time, mirroring the
injectable-env pattern already used in src/lib/memory/typedDecay.ts. No
behavior change for anyone who doesn't set it.
* chore: fix changelog fragment PR number