docs: add ansible howto collection

Four reference files covering:
- ansible.md — core commands, ansible.cfg, key settings
- ansible-inventory.md — static/dynamic inventory, directory layout
- ansible-variables.md — 22-level precedence, scoping, merge behavior
- ansible-roles.md — structure, defaults vs vars, dependencies

TODO.md

@@ -3,7 +3,7 @@
 ## Topics to Write
 
 - [ ] git — common workflows, rebase, stash, bisect
-- [ ] ansible — playbook patterns, inventory, vault
+- [x] ansible — playbook patterns, inventory, vault, variables, roles
 - [ ] podman — build, run, compose, volumes
 - [ ] jq — filters, select, map, slurp
 - [ ] curl — headers, auth, methods, output