docs(build): document real-data workflows

Clarify that `just build` is the production no-bundle launcher build, while
fixture builds are explicit. Document package-directory generation, the
`LANSPREAD_GAMES_DIR` and `--set GAMES_DIR` default-run forms, the metadata
cache behavior, and the force-refresh override so the catalogue workflow is
usable without consulting the recipe implementation.

Test Plan:
- `prettier --check --prose-wrap always --print-width 80 README.md CLAUDE.md` -- passed
- `just --fmt --check` -- passed
- `git diff --cached --check` -- passed

CLAUDE.md

@@ -27,8 +27,14 @@ Top-level `Cargo.toml` pins workspace dependency versions; per-crate
 
 Never use normal cargo ... commands, use the just ... commands instead.
 
-- `just run` — run the GUI in dev mode.
-- `just build` — build the GUI without bundling (also dev mode).
+- `just run` — run the fixture GUI, or real data when `LANSPREAD_GAMES_DIR` is
+  set.
+- `just build` — validate production authority and build the launcher without
+  bundling.
+- `just build-production PACKAGES_DIR` — generate/reuse production authority and
+  build.
+- `just run-fixture` / `just build-fixture` — use the test-only catalogue
+  explicitly.
 - `just fmt` — format the workspace.
 - `just clippy` — lint the workspace.
 - `just test` — run the workspace unit tests.