feat: add source hyperlinks + remove thin from-source.md

Every source reference now links to elixir-lang/elixir at commit f4e1b34.
122 hyperlinks across 11 topic files. Added PATTERN_COMPLETE sentinels.
Removed from-source.md (326 lines, shallow) — covered by existing files.

smells/common-mistakes.md

@@ -1158,3 +1158,5 @@ end
 ```
 
 **Why it's OK here:** There's no conditional logic — every iteration tests the exact same behavior with a trivially predictable result. If one fails, the assertion message includes the specific value. The loop is purely for conciseness.
+
+<!-- PATTERN_COMPLETE -->