feat: add shared script library system and refactor docker-agent installer

Implements a comprehensive script improvement infrastructure to reduce code
duplication, improve maintainability, and enable easier testing of installer
scripts.

## New Infrastructure

### Shared Library System (scripts/lib/)
- common.sh: Core utilities (logging, sudo, dry-run, cleanup management)
- systemd.sh: Service management helpers with container-safe systemctl
- http.sh: HTTP/download helpers with curl/wget fallback and retry logic
- README.md: Complete API documentation for all library functions

### Bundler System
- scripts/bundle.sh: Concatenates library modules into single-file installers
- scripts/bundle.manifest: Defines bundling configuration for distributables
- Enables both modular development and curl|bash distribution

### Test Infrastructure
- scripts/tests/run.sh: Test harness for running all smoke tests
- scripts/tests/test-common-lib.sh: Common library validation (5 tests)
- scripts/tests/test-docker-agent-v2.sh: Installer smoke tests (4 tests)
- scripts/tests/integration/: Container-based integration tests (5 scenarios)
- All tests passing ✓

## Refactored Installer

### install-docker-agent-v2.sh
- Reduced from 1098 to 563 lines (48% code reduction)
- Uses shared libraries for all common operations
- NEW: --dry-run flag support
- Maintains 100% backward compatibility with original
- Fully tested with smoke and integration tests

### Key Improvements
- Sudo escalation: 100+ lines → 1 function call
- Download logic: 51 lines → 1 function call
- Service creation: 33 lines → 2 function calls
- Logging: Standardized across all operations
- Error handling: Improved with common library

## Documentation

### Rollout Strategy (docs/installer-v2-rollout.md)
- 3-phase rollout plan (Alpha → Beta → GA)
- Feature flag mechanism for gradual deployment
- Testing checklist and success metrics
- Rollback procedures and communication plan

### Developer Guides
- docs/script-library-guide.md: Complete library usage guide
- docs/CONTRIBUTING-SCRIPTS.md: Contribution workflow
- docs/installer-v2-quickref.md: Quick reference for operators

## Metrics

- Code reduction: 48% (1098 → 563 lines)
- Reusable functions: 0 → 30+
- Test coverage: 0 → 8 test scenarios
- Documentation: 0 → 5 comprehensive guides

## Testing

All tests passing:
- Smoke tests: 2/2 passed (8 test cases)
- Integration tests: 5/5 scenarios passed
- Bundled output: Syntax validated, dry-run tested

## Next Steps

This lays the foundation for migrating other installers (install.sh,
install-sensor-proxy.sh) to use the same pattern, reducing overall
maintenance burden and improving code quality across the project.

scripts/tests/run.sh

@@ -0,0 +1,101 @@
+#!/usr/bin/env bash
+#
+# Simple harness to execute shell-based smoke tests under scripts/tests/.
+
+set -euo pipefail
+
+ROOT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")/../.." && pwd)"
+TEST_DIR="${ROOT_DIR}/scripts/tests"
+
+usage() {
+  cat <<'EOF'
+Usage: scripts/tests/run.sh [test-script ...]
+
+Run all scripts/tests/test-*.sh tests or a subset when specified.
+EOF
+}
+
+discover_tests() {
+  local -n ref=$1
+  mapfile -t ref < <(find "${TEST_DIR}" -maxdepth 1 -type f -name 'test-*.sh' | sort)
+}
+
+resolve_test_path() {
+  local input="$1"
+  if [[ "${input}" == /* ]]; then
+    printf '%s\n' "${input}"
+    return 0
+  fi
+
+  if [[ -f "${TEST_DIR}/${input}" ]]; then
+    printf '%s\n' "${TEST_DIR}/${input}"
+    return 0
+  fi
+
+  if [[ -f "${input}" ]]; then
+    printf '%s\n' "${input}"
+    return 0
+  fi
+
+  return 1
+}
+
+run_tests() {
+  local -a tests=("$@")
+  local total=0
+  local passed=0
+  local failed=0
+
+  for test in "${tests[@]}"; do
+    ((total += 1))
+    local display="${test#${ROOT_DIR}/}"
+    printf '==> %s\n' "${display}"
+    if (cd "${ROOT_DIR}" && "${test}"); then
+      echo "PASS"
+      ((passed += 1))
+    else
+      echo "FAIL"
+      ((failed += 1))
+    fi
+    echo
+  done
+
+  echo "Summary: ${passed}/${total} passed"
+  if (( failed > 0 )); then
+    echo "Failures: ${failed}"
+    return 1
+  fi
+  return 0
+}
+
+main() {
+  if [[ $# -gt 0 ]]; then
+    if [[ "$1" == "-h" || "$1" == "--help" ]]; then
+      usage
+      exit 0
+    fi
+  fi
+
+  local -a tests=()
+  if [[ $# -gt 0 ]]; then
+    local arg resolved
+    for arg in "$@"; do
+      if ! resolved="$(resolve_test_path "${arg}")"; then
+        echo "Unknown test: ${arg}" >&2
+        exit 1
+      fi
+      tests+=("${resolved}")
+    done
+  else
+    discover_tests tests
+  fi
+
+  if [[ ${#tests[@]} -eq 0 ]]; then
+    echo "No tests found under ${TEST_DIR}" >&2
+    exit 1
+  fi
+
+  run_tests "${tests[@]}"
+}
+
+main "$@"