Declare the repo's expert (NORTE R6) #807

Problem

agentic-board has no declared repo expert: there is no .claude/agents/experto.md and no FICHA.md. Under the NORTE rule R6, a tool repo's code is reviewed and merged by its expert. Without one, every PR stalls on "who merges?". #806 hit this: claude-review passed, but nobody owned the merge into next. The /board work step that lets a repo-declared expert merge (#804) also has nothing to read here.

Minimum to decide

1. Who the expert is: a .claude/agents/experto.md agent definition, and the model it runs on.
2. Scope: the plugin code under plugins/agentic-board/ (scripts, skills, commands, hooks), the tests and the workflows. Out of scope: visible output that Cristóbal approves by looking at it (README visuals, the website).
3. Merge conditions: Pester (Windows and ubuntu) green, a claude-review on the head commit with no blocking findings, no unresolved threads, and the target branch. While the directory listing tracks main in first review, the expert merges into next only. Promoting next to main/stable stays a release decision, per the plugin-directory change protocol.
4. What the expert never does: push or merge to main during review, move stable, or touch repo secrets. A failing PROJECTS_TOKEN (board-sync 401) goes to the owner.
5. FICHA.md: the one-page card NORTE expects (purpose, owner, expert, gates), drafted for Cristóbal's approval.

Done when

experto.md and FICHA.md are merged into next, and Test-Norte.ps1 reports this repo as compliant on R6.