docs: add the user-facing MkDocs documentation site

Build a complete user-facing documentation site (MkDocs Material) under docs/, served at roboco.dev/docs via a new gh-pages deploy workflow.

- Sections: Get Started, The Company, the Tour, Operating the Panel, Choosing & Running Models, Cost & Observability, Optional Subsystems, Configure & Deploy, API Reference, Troubleshooting & Security (55 pages).
- mkdocs.yml (Material theme; excludes the agent-facing rag/ corpus, internal scratch, and orphaned stub trees) and .github/workflows/docs.yml (mkdocs gh-deploy to gh-pages).
- Retire the stale root usage.md and deployment.md to redirect stubs into the site.
- Fix the docs tooling: add the pymarkdownlnt dependency + .pymarkdown.json, run serve-docs/lint-docs/fix-docs under the docs extra, add a build-docs strict gate.
- Fix the roboco console-script entry point (cli, not the un-awaited async main).
- README: correct the project-structure tree (optimal.py, alembic) and link the docs site.

docs/how-to/01-the-company.md

@@ -4,7 +4,7 @@ What keeps twenty-two agents from dissolving into noise is that RoboCo is relent
 
 Work in RoboCo is always a **task**, and tasks nest into a tree that mirrors the org itself:
 
-```
+```text
 CEO (you, the human)
  ├── Intake (on-demand interviewer — drafts a task with you)
  ├── Secretary (your chief-of-staff — acts only on your command)