fix(ci): truncate Discord commit field to avoid 400 on long messages

[TomGrobbe]

Problem

The Discord Notification job failed on the latest master CI/CD run (run 27018095845) with:

curl: (22) The requested URL returned error: 400

The build, release, and tag all succeeded (v3.8.14 published fine); only the notification step broke.

Root cause

The "Commit" embed field was built from the full commit message (github.event.head_commit.message). For the squash-merged commit 0df96a4, that message is 975 characters (it includes the entire PR body plus the co-author trailer). Combined with the [`sha`](commit_url) — link prefix (about 96 chars), the field value reached roughly 1071 characters, which exceeds Discord's 1024-character limit for embed field values. Discord rejects the payload with HTTP 400, and curl's -f flag turns that into exit code 22.

Fix

• Use only the commit subject (first line) instead of the whole message.
• Truncate it to 256 chars as a safety cap (with an ellipsis), so the payload can never exceed the limit.
• Pass the message via the COMMIT_MSG environment variable instead of interpolating ${{ github.event.head_commit.message }} directly into the shell script. The previous direct interpolation would also break the script if a commit subject ever contained a double quote (a latent shell-injection / quoting bug).

No other steps were changed.