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.

scripts/dogfood-build.sh

@@ -1,9 +1,16 @@
 #!/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.
-# Usage: bash scripts/dogfood-build.sh
-# On success: prints the verified binary path. Use as:
-#   CLAW=$(bash scripts/dogfood-build.sh) && $CLAW version --output-format json
+# Suppresses Cargo compile noise on stderr.
+# Prints the verified binary path on success. Use as:
+#
+#   CLAW=$(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 "${BASH_SOURCE[0]}")/.." && pwd)"
@@ -11,15 +18,25 @@ RUST_DIR="$REPO_ROOT/rust"
 BINARY="$RUST_DIR/target/debug/claw"
 EXPECTED_SHA="$(git -C "$REPO_ROOT" rev-parse --short HEAD)"
 
-echo "▶ Building claw from $REPO_ROOT ($(git -C "$REPO_ROOT" log --oneline -1))..." >&2
-# Inject GIT_SHA so that version --output-format json returns a non-null sha.
-# Without this, option_env!("GIT_SHA") in main.rs always yields None → null.
-GIT_SHA="$EXPECTED_SHA" cargo build \
-    --manifest-path "$RUST_DIR/Cargo.toml" \
-    -p rusty-claude-cli -q
+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 "✗ Build succeeded but binary not found at $BINARY" >&2
+    echo "✗ Binary not found at $BINARY" >&2
     exit 1
 fi
 
@@ -29,18 +46,23 @@ BINARY_SHA=$("$BINARY" version --output-format json 2>/dev/null \
 
 if [[ "$BINARY_SHA" == "null" || -z "$BINARY_SHA" ]]; then
     echo "✗ Provenance check failed: binary reports git_sha: null" >&2
-    echo "  Ensure GIT_SHA is passed to cargo build (this script does it automatically)." >&2
-    echo "  Binary: $BINARY" >&2
     exit 1
 fi
 
 if [[ "$BINARY_SHA" != "$EXPECTED_SHA" ]]; then
     echo "✗ Provenance mismatch: binary=$BINARY_SHA, HEAD=$EXPECTED_SHA" >&2
-    echo "  Rerun after 'git pull' or check for uncommitted changes." >&2
     exit 1
 fi
 
-echo "✓ Binary verified: $BINARY_SHA == HEAD ($EXPECTED_SHA)" >&2
-echo "  cargo run alternative: ~1s overhead per invocation (vs 7ms for pre-built)" >&2
-echo "  To dogfood: export CLAW=$BINARY" >&2
+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"