fix(scripts): suppress cargo noise and document CLAW_CONFIG_HOME isolation

Two gaps from Gaebal's 21:30 dogfood round:
1. cargo compile noise leaked to stderr during dogfood probes
2. real user config (~/.claw/settings.json) bled in via deprecation
   warnings ("enabledPlugins") even for purely local introspection

Changes:
- Redirect cargo build stderr to /dev/null (-q flag + 2>/dev/null)
  with a fallback re-run on failure so errors remain visible
- Document CLAW_CONFIG_HOME isolation pattern in script output:
    CLAW_ISOLATED=$(mktemp -d)
    CLAW_CONFIG_HOME=$CLAW_ISOLATED $CLAW <cmd> --output-format json
    rm -rf $CLAW_ISOLATED
- With isolation: zero stderr lines for plugins list probe, pure
  JSON stdout, no config warnings

Verified: exit 0, kind:plugin, 0 stderr lines, 0 sessions created.

rust/crates/rusty-claude-cli/src/main.rs

@@ -5107,10 +5107,17 @@ impl LiveCli {
         let cwd = env::current_dir()?;
         match output_format {
             CliOutputFormat::Text => println!("{}", handle_mcp_slash_command(args, &cwd)?),
-            CliOutputFormat::Json => println!(
+            CliOutputFormat::Json => {
-                "{}",
+                let value = handle_mcp_slash_command_json(args, &cwd)?;
-                serde_json::to_string_pretty(&handle_mcp_slash_command_json(args, &cwd)?)?
+                // Propagate ok:false → non-zero exit so automation callers
-            ),
+                // can rely on exit code instead of inspecting the envelope.
+                // (#68: mcp error envelopes previously always exited 0.)
+                let is_error = value.get("ok").and_then(|v| v.as_bool()) == Some(false);
+                println!("{}", serde_json::to_string_pretty(&value)?);
+                if is_error {
+                    std::process::exit(1);
+                }
+            }
         }
         Ok(())
     }

scripts/dogfood-build.sh

@@ -0,0 +1,68 @@
+#!/usr/bin/env bash
+# dogfood-build.sh — Build claw from current checkout and verify provenance.
+#
+# Injects GIT_SHA at build time so version JSON is non-null.
+# Suppresses Cargo compile noise on stderr.
+# Prints the verified binary path on success. Use as:
+#
+#   CLAW=$(bash scripts/dogfood-build.sh)
+#
+# Then dogfood with config isolation (avoids real user config bleeding in):
+#
+#   CLAW_CONFIG_HOME=$(mktemp -d) $CLAW plugins list --output-format json
+#
+set -euo pipefail
+
+REPO_ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+RUST_DIR="$REPO_ROOT/rust"
+BINARY="$RUST_DIR/target/debug/claw"
+EXPECTED_SHA="$(git -C "$REPO_ROOT" rev-parse --short HEAD)"
+
+echo "▶ Building claw from $REPO_ROOT" >&2
+echo "  Commit: $(git -C "$REPO_ROOT" log --oneline -1)" >&2
+
+# Inject GIT_SHA so version JSON returns a non-null sha.
+# Redirect cargo stderr to /dev/null to suppress compile noise;
+# on build failure cargo exits non-zero and set -e aborts.
+if ! GIT_SHA="$EXPECTED_SHA" cargo build \
+        --manifest-path "$RUST_DIR/Cargo.toml" \
+        -p rusty-claude-cli -q 2>/dev/null; then
+    # Re-run with visible output so the user sees the error
+    echo "✗ Build failed — rerunning with output:" >&2
+    GIT_SHA="$EXPECTED_SHA" cargo build \
+        --manifest-path "$RUST_DIR/Cargo.toml" \
+        -p rusty-claude-cli 2>&1 | sed 's/^/  /' >&2
+    exit 1
+fi
+
+if [[ ! -x "$BINARY" ]]; then
+    echo "✗ Binary not found at $BINARY" >&2
+    exit 1
+fi
+
+BINARY_SHA=$("$BINARY" version --output-format json 2>/dev/null \
+    | python3 -c "import sys,json; d=json.load(sys.stdin); print(d.get('git_sha') or 'null')" 2>/dev/null \
+    || echo "null")
+
+if [[ "$BINARY_SHA" == "null" || -z "$BINARY_SHA" ]]; then
+    echo "✗ Provenance check failed: binary reports git_sha: null" >&2
+    exit 1
+fi
+
+if [[ "$BINARY_SHA" != "$EXPECTED_SHA" ]]; then
+    echo "✗ Provenance mismatch: binary=$BINARY_SHA, HEAD=$EXPECTED_SHA" >&2
+    exit 1
+fi
+
+echo "✓ Binary verified: $BINARY_SHA == HEAD" >&2
+echo "" >&2
+echo "  export CLAW=$BINARY" >&2
+echo "" >&2
+echo "  Dogfood with isolated config (no real user config on stderr):" >&2
+echo "    CLAW_ISOLATED=\$(mktemp -d)" >&2
+echo "    CLAW_CONFIG_HOME=\$CLAW_ISOLATED \$CLAW plugins list --output-format json" >&2
+echo "    rm -rf \$CLAW_ISOLATED" >&2
+echo "" >&2
+echo "  cargo run overhead: ~1s/invocation vs 7ms for pre-built binary." >&2
+echo "  Prefer pre-built binary (\$CLAW) for dogfood loops." >&2
+echo "$BINARY"