Avoid duplicate config warnings for JSON consumers (#3190)

JSON config output already carries collected config diagnostics in warnings[], so prose stderr emission must be reserved for text/local paths. Lazy permission-mode default resolution prevents an earlier config load from leaking the same deprecation before the JSON renderer runs.\n\nConstraint: ROADMAP #815 requires text mode to keep human stderr warnings while JSON config/list suppresses duplicate app-level config prose.\nRejected: Filtering all stderr in JSON mode | would hide cargo/compiler or unrelated diagnostics outside the app config warning path.\nConfidence: high\nScope-risk: narrow\nDirective: Keep load_collecting_warnings side-effect-free; use load() for human stderr emission.\nTested: cargo fmt; cargo test -p rusty-claude-cli --test output_format_contract config_json_reports_deprecations_structurally_without_stderr_duplicate_815; cargo test -p rusty-claude-cli --test output_format_contract; manual target/debug/claw JSON config fixture.\nNot-tested: cargo clippy -p rusty-claude-cli --all-targets -- -D warnings is blocked by pre-existing runtime dead_code/trident warnings.

rust/crates/runtime/src/config.rs

@@ -379,12 +379,6 @@ impl ConfigLoader {
             loaded_entries.push(entry);
         }
 
-        // Still emit to stderr for non-JSON callers that go through the normal load() path;
-        // here we just *also* return them so callers can surface them structurally.
-        for warning in &all_warnings {
-            emit_config_warning_once(warning);
-        }
-
         let merged_value = JsonValue::Object(merged.clone());
 
         let feature_config = RuntimeFeatureConfig {