fix(release): accept the org signing seed, not only seed+pub

[Jaro-c]

#32 alone won't let unitpm release. It points the workflow at GLYNDOR_RELEASE_ED25519_KEY and embeds HFv7… — both correct — but the signer refuses that secret:

$ RELEASE_SIGNING_KEY=<32-byte seed> go run scripts/sign.go file
key size 32, want 64
exit status 1

sign.go required a 64-byte seed+pub. The org secret is a raw 32-byte seed — that's what podup's sign.py reads (must decode to 32 bytes) and what signed podup 1.9.1 an hour ago. So the sign step dies and the release never publishes.

This is very likely why unitpm drifted onto a key of its own (3eSCGskG…) while the docs described one shared key: the shared key was unusable here. Not carelessness — incompatibility.

Accepts both forms: ed25519.NewKeyFromSeed expands a 32-byte seed to the 64-byte private key, and the 64-byte form still works untouched, so nothing existing breaks. A wrong length is still refused.

Verified by running it, not reading it (Go 1.26.4, locally):

input	before	after
32-byte seed (the real secret)	, exit 1	signs — and the signature verifies against the public key derived from that seed, which is the check performs in the field
64-byte seed+pub	signs	signs
48 bytes	rejected	, exit 1

gofmt, go vet, go build all clean.

Wants to land together with #32 — one without the other still leaves unitpm unable to cut a release.