Suggest brand IDs after brand: and place unknown-brand errors on their line

- Completion for brand values in .archify (YAML) and Archify JSON sources,
  from the bundled brand-marks/catalog.json (107 marks); aliases and
  product names match, so "kafka" offers apache-kafka
- brand/unknown diagnostics carry their JSON pointer only at the start of
  the message; use it when subject.path is missing, so the error lands on
  the brand line instead of the top of the file
- Unit tests for catalog loading, value detection and pointer fallback;
  integration checks for completion and diagnostic placement

Closes #5

CHANGELOG.md

@@ -1,5 +1,10 @@
 # Changelog
 
+## Unreleased
+
+- **Brand completion:** after `brand:` in `.archify` files and Archify JSON sources, the editor suggests the 107 built-in brand IDs with their product names. Aliases and product names match too (`kafka` → `apache-kafka`, `postgres` → `postgresql`).
+- **Unknown brands are underlined on their own line.** Archify reports these without a location, so they used to appear at the top of the file.
+
 ## 0.4.1 — 2026-10-04
 
 - The new-diagram command is now titled **New Diagram Archify…** everywhere, including the Explorer folder menu, where it read *New Diagram…*.