chore(license): switch from MIT to GPL-3.0-or-later

Adds top-level LICENSE (GPL-3.0 full text), updates README, and sets
`license = "GPL-3.0-or-later"` on all four crate manifests.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/relicario-server/Cargo.toml

@@ -2,6 +2,8 @@
 name = "relicario-server"
 version = "0.1.0"
 edition = "2021"
+description = "Pre-receive Git hook for relicario password manager"
+license = "GPL-3.0-or-later"
 
 [dependencies]
 relicario-core = { path = "../relicario-core" }