feat: generate brain tools skill from OpenAPI; rename db-yaml to postgres (#21)

Cursor skills stay in lockstep with serve handlers. CI checks every bin/
path named in SKILL.md exists.

skills/postgres/SKILL.md

@@ -0,0 +1,39 @@
+---
+name: postgres
+description: >-
+  Read Postgres as compact YAML through bin/postgres/query.go (read-only
+  guard, named profiles). Use when a task needs table contents, column types,
+  or a SELECT against an ops database.
+---
+
+# postgres
+
+`bin/postgres/query.go` wraps vendored `bin/db/psql-yq`. Output is YAML
+(cheaper than psql ASCII, easy to slice with `yq`).
+
+```bash
+bin/postgres/query.go --profile onlyoffice -s document_asset   # column list
+bin/postgres/query.go --profile onlyoffice -t task_result -l 20  # sample rows
+bin/postgres/query.go --profile onlyoffice -c 'SELECT ...'     # query → YAML
+```
+
+Ad-hoc targets without a profile:
+
+```bash
+bin/postgres/query.go --container my-pg --db app -c 'SELECT 1'
+bin/postgres/query.go --dsn 'postgres://user@host:5432/db' -c 'SELECT 1'
+```
+
+## Profiles
+
+Connection details live in `$HOME/.config/brain/db-profiles.yml` (mode 600),
+never in a project repo. A profile names either a `container` or a `host`;
+passwords are read from a separate `password_env_file` and never appear in argv.
+
+## Rules
+
+- **Read-only.** Any `insert|update|delete|drop|truncate|alter|create|grant|
+  revoke|vacuum|copy` is rejected with exit 3. Do not work around it.
+- **PII.** Client CRM databases: aggregate and count freely; never copy names
+  or addresses into chat, issues or docs.
+- Use `-l` to keep samples small. Twenty rows answer most questions.