chore: merge elixir-conventions and oban-conventions into sources/

Absorbed content from rodin/elixir-conventions and rodin/oban-conventions
into a sources/ directory. These are reference material — descriptive,
not prescriptive. Patterns that prove broadly applicable get promoted
into patterns/.

Part of taxonomy cleanup (issue #4):
- Pattern = prescriptive, follow these
- Convention/Source = reference, study for ideas

The original repos can now be archived.

sources/README.md

@@ -0,0 +1,15 @@
+# Sources
+
+Reference material extracted from specific projects. Study for ideas, don't copy blindly.
+
+These are **descriptive** — they document what a project does and why.
+The `patterns/` directory is **prescriptive** — it tells you what to do.
+
+Patterns that prove broadly applicable get promoted from here into `patterns/`.
+The rest stays as reference for understanding how mature projects solve specific problems.
+
+## Files
+
+- `elixir-lang.md` — conventions from the elixir-lang/elixir source
+- `elixir-lang-analysis.md` — deeper analysis of elixir-lang source architecture
+- `oban.md` — patterns from oban-bg/oban (job processing, plugin system)