feat(security): add gitleaks pre-commit scan, wire into task check (infra#39)

Ported from mathias/infra's .gitleaks.toml (2026-08-04) -- same
homelab bearer/MCP-token rules, plus a BRAIN_PG_DSN pattern for the
hyperguild#20 leak class. Triaged 10 initial findings: 9 were
claudewatcher's own scrubber test fixtures (deliberately fake), 1 was
gitleaks matching the literal placeholder word "REDACTED" in a plan
doc. Both allowlisted with rationale, clean scan confirmed before
wiring into `task check`.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016FQX4RwxyxuQY4yf4XaACL

Taskfile.yml

@@ -101,6 +101,37 @@ tasks:
       - task: lint
       - task: test
       - task: vet
+      - task: security:gitleaks
+
+  # ── Security ─────────────────────────────────────────────
+  security:gitleaks:
+    desc: Scan the working tree for secrets (gitleaks, fail-closed; skipped if gitleaks absent)
+    dir: '{{.ROOT_DIR}}'
+    cmds:
+      - |
+        GL="$(command -v gitleaks || true)"
+        [ -z "$GL" ] && [ -x "$(go env GOPATH 2>/dev/null)/bin/gitleaks" ] && GL="$(go env GOPATH)/bin/gitleaks"
+        if [ -z "$GL" ]; then
+          echo "⚠ gitleaks not installed — skipping secret scan (CI enforces it)."
+          echo "  Install: go install github.com/zricethezav/gitleaks/v8@latest"
+          exit 0
+        fi
+        "$GL" detect --no-git --redact --config .gitleaks.toml --source .
+
+  security:gitleaks:history:
+    desc: "One-time FULL-HISTORY secret audit (infra#39 rotation pass; not a per-push gate)"
+    dir: '{{.ROOT_DIR}}'
+    cmds:
+      - |
+        GL="$(command -v gitleaks || true)"
+        [ -z "$GL" ] && [ -x "$(go env GOPATH 2>/dev/null)/bin/gitleaks" ] && GL="$(go env GOPATH)/bin/gitleaks"
+        if [ -z "$GL" ]; then
+          echo "gitleaks not installed: go install github.com/zricethezav/gitleaks/v8@latest" >&2
+          exit 2
+        fi
+        echo "Scanning FULL git history (redacted). Known historical leaks are expected"
+        echo "until the infra#39 rotation pass completes — triage against the rotation list."
+        "$GL" detect --redact --config .gitleaks.toml
 
   lint:
     cmds: