feat: add Kubernetes DNS support and unify example setup scripts

[miguelgila]

Summary

• Kubernetes DNS resolution for Reaper pods: New REAPER_DNS_MODE env var (kubernetes/k8s) writes the kubelet-prepared resolv.conf into the overlay namespace, enabling Reaper workloads to resolve Kubernetes service names (e.g., my-svc.default.svc.cluster.local) via CoreDNS
• Unified example setup scripts: All 8 example setup.sh scripts now default to downloading published release binaries, with a --build flag for source compilation. Added --help/-h to all scripts.
• Example 08 updated: Replaced hardcoded clusterIP: 10.96.100.100 with DNS-based service discovery, demonstrating the new DNS feature

Changes

Core (overlay.rs, main.rs)

• DnsMode enum + read_dns_config() reads REAPER_DNS_MODE env var
• apply_kubernetes_dns() finds /etc/resolv.conf OCI mount, reads content via /proc/1/root/<source>, writes it into the overlay
• Called from do_start() after volume mounts, before workload spawn
• DNS failure is fatal when explicitly opted-in (misconfiguration should not silently fall back)

Ansible (install-reaper.yml)

• Adds REAPER_DNS_MODE=kubernetes to /etc/default/containerd (default for Ansible-deployed clusters)

Integration tests

• New test_kubernetes_dns_resolution() verifying Reaper pods can resolve kubernetes.default.svc.cluster.local and a custom ClusterIP service

Examples (01-08)

• All setup.sh scripts: added --help, --build, release version resolution via scripts/lib/release-utils.sh
• Example 08: DNS-based LDAP service discovery instead of hardcoded ClusterIP

Test plan

• cargo test — unit tests pass (DNS config parsing)
• cargo clippy --target x86_64-unknown-linux-gnu --all-targets — clean
• bash -n syntax check on all 8 setup.sh scripts
• ./scripts/run-integration-tests.sh — full suite including new test_kubernetes_dns_resolution
• Manual: run example 08 with --build flag, verify getent passwd user1 resolves via DNS

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.44%. Comparing base (eaa34f4) to head (7b6c75e).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #21      +/-   ##
==========================================
+ Coverage   86.20%   86.44%   +0.23%     
==========================================
  Files           4        4              
  Lines         174      177       +3     
==========================================
+ Hits          150      153       +3     
  Misses         24       24              

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