[ADD] odoo-at-payroll: wissensbasis skill + WIKU source integration model

Consolidate the binding knowledge-base rules (layer architecture,
licence decision D1, id freeze, D2 frontmatter schema, curation
conventions, batch workflow, validation gates, intake pitfalls)
from README/RUNBOOK/MEMORY into a dedicated agent skill so every
new thread reads them before planning Wissensbasis work.

- new licensed source corpus .wiku/ (WIKU Personal publications:
  Fachbroschueren, Arbeitsunterlagen, Casebooks, 'WIKU Personal
  aktuell'; gitignored like .lexis360/, never committed)
- integration model: one shared corpus with a single kb.json, ids
  wk-<prefix>-<nn> on the existing cluster map, the work field
  distinguishing the sources, Layer-2 filenames prefixed
  wiku_<slug>.md; pipeline to be a multi-source extension of
  build_lexis_kb.py (no fork) -- follow-up after batch 7
- AGENTS.md skill selection gains the wissensbasis entry

AGENTS.md

@@ -32,6 +32,9 @@ no skill applies.
 - Any payroll work (besoldung, Lohnsteuer, SV, Meldewesen, annual value
   updates, extending the modules in this repo):
   `payroll/SKILL.md`
+- Any Wissensbasis work (batch intake, curation, `kb.json`, the sources
+  `.lexis360/` and `.wiku/`):
+  `wissensbasis/SKILL.md`
 
 Multiple skills may apply. Read all applicable skills.