Add CLI args parsing tests and lib target; fix summary string building

Introduce `src/lib.rs` and library target so integration tests can import `docx_mcp`. Add focused `tests/args_tests.rs` verifying clap flag/env parsing and `SecurityConfig::from_args`/`from_env`. Enable clap `env` feature and guard the binary behind a `build-bin` feature to allow testing without unresolved MCP server deps. Fix `get_summary` to build owned strings safely.

These changes ensure argument options work correctly and are covered by comprehensive tests, independent of heavier integration suites.

.claude/settings.local.json

@@ -2,7 +2,11 @@
   "permissions": {
     "allow": [
       "Bash(chmod:*)",
-      "Bash(cargo build:*)"
+      "Bash(cargo build:*)",
+      "Bash(rustc:*)",
+      "Bash(cargo check:*)",
+      "Bash(git push:*)",
+      "Bash(rm:*)"
     ],
     "deny": []
   }