Add pdbmux: a merging PuppetDB proxy for the VM->k8s migration

During the VM->k8s Puppet migration there are two PuppetDBs - the legacy
Consul-registered one (http://puppetdbapi.service.consul:8080) and the new
k8s one (https://puppetdb.k8s.syd1.au.unkin.net) - and nodes move between
them as they migrate. node-lookup and pblastreport need a single, consistent
merged view without knowing which PuppetDB a node currently lives in.

This adds pdbmux, a small HTTP daemon that fronts both backends:

- Adds cmd/pdbmux/ (config.go, merge.go, server.go, main.go): a cobra tool
  whose default action (also `serve`) starts the proxy, plus config init/show
  and version subcommands, following the repo's config precedence pattern
  (defaults < config file < env PDBMUX_* < flags).
- Merges GET /pdb/query/v4/nodes: dedupes by certname, keeping the record with
  the newer report_timestamp.
- Merges GET /pdb/query/v4/facts at node granularity: keeps all facts from the
  backend owning each certname, chosen by the freshness strategy (per-certname
  report_timestamp map from /nodes, cached for freshness_ttl) or a static
  prefer-backend fallback.
- Fans out to both backends concurrently, serves the survivor if one fails, and
  returns 502 only when both fail; passes records through as raw JSON so unknown
  fields survive.
- Transparently proxies any other /pdb/query/v4/* path to the configurable
  primary, and exposes /healthz with per-backend reachability (200 ok /
  200 degraded / 503 down).
- Adds table-driven tests (go test -race, no network) covering merge logic,
  handler behaviour with httptest backends, query passthrough, one/both backend
  down, and config precedence/validation.
- Wires pdbmux into the build/release: Makefile BINARIES, scripts/build-rpm.sh,
  nfpm packaging (binary + completions + a systemd unit), and the release
  pipeline's cross-platform build + Gitea asset list.
- Documents pdbmux (what/why/endpoints/merge-semantics/config/deployment) in a
  new README.md and updates AGENTS.md.

packaging/nfpm.yaml

@@ -42,6 +42,18 @@ contents:
       mode: 0755
       owner: root
       group: root
+  - src: dist/pdbmux
+    dst: /usr/bin/pdbmux
+    file_info:
+      mode: 0755
+      owner: root
+      group: root
+
+  # systemd unit for the pdbmux proxy daemon (config: /etc/pdbmux/config.yaml).
+  - src: packaging/pdbmux.service
+    dst: /usr/lib/systemd/system/pdbmux.service
+    file_info:
+      mode: 0644
 
   # Shell completions (generated by scripts/build-rpm.sh before packaging).
   - src: dist/completions/node-lookup.bash
@@ -80,3 +92,15 @@ contents:
     dst: /usr/share/fish/vendor_completions.d/pblastreport.fish
     file_info:
       mode: 0644
+  - src: dist/completions/pdbmux.bash
+    dst: /usr/share/bash-completion/completions/pdbmux
+    file_info:
+      mode: 0644
+  - src: dist/completions/_pdbmux
+    dst: /usr/share/zsh/site-functions/_pdbmux
+    file_info:
+      mode: 0644
+  - src: dist/completions/pdbmux.fish
+    dst: /usr/share/fish/vendor_completions.d/pdbmux.fish
+    file_info:
+      mode: 0644

packaging/pdbmux.service

@@ -0,0 +1,31 @@
+[Unit]
+Description=pdbmux - merging proxy over old + new PuppetDB during migration
+Documentation=https://git.unkin.net/unkin/node-lookup
+After=network-online.target
+Wants=network-online.target
+
+[Service]
+Type=simple
+# Configure via /etc/pdbmux/config.yaml or PDBMUX_* env in this file / a drop-in.
+# Example env overrides:
+#   Environment=PDBMUX_LISTEN=:8080
+#   Environment=PDBMUX_MERGE=freshness
+Environment=XDG_CONFIG_HOME=/etc
+ExecStart=/usr/bin/pdbmux serve
+Restart=on-failure
+RestartSec=5
+
+# Hardening: pdbmux only makes outbound HTTP(S) and listens on a socket.
+DynamicUser=yes
+NoNewPrivileges=yes
+ProtectSystem=strict
+ProtectHome=yes
+PrivateTmp=yes
+PrivateDevices=yes
+ProtectKernelTunables=yes
+ProtectControlGroups=yes
+RestrictAddressFamilies=AF_INET AF_INET6
+RestrictNamespaces=yes
+
+[Install]
+WantedBy=multi-user.target