feat(cli): relicario add login with flag + interactive prompting

Unlocks vault, builds LoginCore from flags (password via rpassword if
--password-prompt), saves item + manifest, commits via hardened git.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

crates/relicario-cli/Cargo.toml

@@ -22,6 +22,7 @@ rand = "0.8"
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 zeroize = "1"
+url = "2"
 
 [dev-dependencies]
 assert_cmd = "2"