chore: strictly deny unused imports workspace-wide

- Added `unused_imports = "deny"` to workspace lints in Cargo.toml
- Opted-in all crates to inherit workspace lints
- Fixed resulting unused imports in `dump_schema.rs`

server/src/bin/dump_schema.rs

@@ -1,4 +1,4 @@
-use mcp_memory_server::tools::{DecisionsTool, TechDebtTool, ManageCheckpointTool, HypothesesTool, AgentSignalsTool};
+use mcp_memory_server::tools::AgentSignalsTool;
 use schemars::schema_for;
 
 fn main() {