fix: remove magika/ort dependency (CI glibc too old for C23 symbols)

Root cause: ort (ONNX Runtime) links against __isoc23_strtoll which
requires glibc 2.38+. CI runner has older glibc, causing linker failure.

Replace magika ML detection with regex-only ContentRouter.
Regex fallback already covers all content types (JSON, log, diff, code).
All 294 tests passing.

crates/mem-core/Cargo.toml

@@ -16,8 +16,8 @@ sha2 = { workspace = true }
 tracing = { workspace = true }
 hex = "0.4"
 time = { version = "0.3", features = ["serde", "formatting", "parsing", "macros"] }
-magika = "1.1.0"
-ort = { version = "2.0.0-rc.12", default-features = true }
+# magika and ort removed: CI runner glibc too old for ort's C23 symbols
+# regex fallback in router.rs covers all content types
 regex = "1.10"
 once_cell = "1.19"
 indexmap = "2.0"