fix(deploynotify): read commit SHA from Coolify SOURCE_COMMIT runtime env

Baking SOURCE_COMMIT as a Docker build arg never worked: Coolify exposes it
as a runtime env var, not a build arg, so gitSHA was always empty and the owner
DM was skipped. Read SOURCE_COMMIT from the container env at startup (falling
back to the ldflags-baked gitSHA for local builds), forward it via the compose
environment, and drop the dead build-arg baking.

.env.example

@@ -23,8 +23,9 @@ ADMIN_IDS=
 # Only the twentyq module needs this. Leave blank to disable that command.
 GEMINI_API_KEY=
 
-# SOURCE_COMMIT is injected as a build arg by Coolify (it sets it automatically)
-# for the deploynotify owner DM. Leave unset for local `docker compose up` —
+# SOURCE_COMMIT is injected into the container at runtime by Coolify (a
+# predefined env var holding the commit SHA); read at startup for the
+# deploynotify owner DM. Leave unset for local `docker compose up` —
 # deploynotify just stays silent.
 # SOURCE_COMMIT=