feat: DuckDB quantiles in-process (gcc CGO), not Ladybug. (#34)

OQ3: internal/duckstats + bin/qa/stats.go. Zig stays Ladybug-only.
mikefarah/yq for small structured slices. Gitea #30.

skills/yq/SKILL.md

@@ -0,0 +1,33 @@
+---
+name: yq
+description: >-
+  Use https://github.com/mikefarah/yq to work with YAML, JSON, XML, CSV,
+  TOML, HCL where it's efficient and less code. Use when slicing compose,
+  config, --json tool output, CSV/TOML/HCL/XML, or converting between those
+  formats. Not kislyuk Python yq. Not jq when yq already does the job.
+---
+
+# yq (mikefarah)
+
+Use https://github.com/mikefarah/yq to work with YAML, JSON, XML, CSV, TOML, HCL where it's efficient and less code.
+
+This is the Go `yq` (`yq --version` contains `mikefarah`). It is not
+kislyuk/yq (Python, jq-syntax, YAML-only wrapper). `bin/db/psql-yq` already
+calls this binary.
+
+Prefer `yq` over `python3 -c`, `jq`, or ad-hoc parsers when one expression
+reads or converts the file. Keep Python/Go for HTTP, binary protocols, and
+in-process tests.
+
+```bash
+yq '.services.picoclaw.image' compose.yaml
+yq -P . deploy/picoclaw/config.json          # JSON → YAML
+yq -o=json '.gates'                           # JSON stdin (qa/system_perf.py --json)
+yq -p=csv -o=json .
+yq -p=xml -o=json .
+yq -p=toml '.package.name' file.toml
+bin/brain/search.go "LadybugDB" --json | yq '.[].ref'
+bin/web/search.go "hypervisor" --json | yq -r '.results[].url'
+```
+
+Do not print secrets, PII, or `$HOME/.config/brain/` through `yq`.