docs(meta): document model selection and 1M context option

Add model selection section to CLAUDE.md covering /model sonnet[1m]
and opus[1m] for extended context sessions.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

CLAUDE.md

@@ -154,6 +154,15 @@ Key rules:
 Use conventional commits with project-specific scopes:
 `agents`, `skills`, `docs`, `briefings`, `discussions`, `schema`, `db`, `config`, `engine`, `simulation`, `client`, `ui`, `audio`, `assets`, `meta`
 
+### Model selection
+
+Default model is Opus 4.6 (200K context). For heavy sessions (workshops,
+sprint planning, large reviews), switch to extended context on-demand:
+
+- `/model sonnet[1m]` — Sonnet 4.6 with 1M context window
+- `/model opus[1m]` — Opus 4.6 with 1M context window
+- Cost: 2x input + 1.5x output for tokens beyond 200K (Tier 4 required)
+
 ### Pull requests
 **Use `tea` (Gitea CLI), not `gh` (GitHub CLI).** The remote is Gitea at `git.schweitz.internal`.