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.

docs/deploy-coolify-selfhosted.md

@@ -88,10 +88,11 @@ SSM/AWS lookup that fails with no AWS creds and bricks startup), `KV_PROVIDER`,
    bot token; a second poller gets HTTP 409, and a second in-process scheduler
    double-fires crons. Prefer **stop-first redeploys** so two containers never
    overlap near a cron time.
-5. **Build arg for deploynotify:** Coolify sets `SOURCE_COMMIT` automatically
-   and the compose build forwards it, so the owner gets the "new version" DM
-   with no manual wiring. Without it, `deploynotify` stays silent (no crash) —
-   but you lose that notification.
+5. **deploynotify commit SHA:** Coolify injects `SOURCE_COMMIT` (a predefined
+   runtime env var) into the container, and the compose `environment` forwards
+   it, so the bot reads it at startup and DMs the owner the "new version"
+   notice — no manual wiring. Without it, `deploynotify` stays silent (no
+   crash) — but you lose that notification.
 6. **Health check:** use Coolify's HTTP monitor against `GET /` (returns
    `text/plain` `miti99bot ok`). Do **not** use a compose `healthcheck` — the
    distroless image has no shell/curl and `cmd/server` has no `-healthcheck`