fix(deps): move zstd to 0.14 (RUSTSEC-2026-0391/0398/0403)

The workspace pin goes from 0.13 to 0.14, which brings zstd-safe 8.2.
tonic 0.14.9 follows onto zstd 0.14, so 0.13 leaves the lock. kigi-shell
declared zstd but never used it; the declaration goes.

crates/codegen/kigi-shell/Cargo.toml

@@ -146,7 +146,6 @@ kigi-http = { workspace = true }
 kigi-models = { workspace = true }
 flate2 = { workspace = true }
 fs2 = { workspace = true }
-zstd = { workspace = true }
 tar = { workspace = true }
 url = { workspace = true }
 kigi-secrets = { path = "../kigi-secrets" }