Enable Elixir tracking and update .gitignore

- Add .gitattributes for Elixir file tracking. - Extend .gitignore for Elixir-related and other development files. -
Introduce Agents.md with project guidelines and usage rules. - Create CLAUDE.md as a symlink to Agents.md. - Update
mix.exs to specify proper version for elixir_sense.

.gitignore

@@ -1,3 +1,19 @@
+# Development artefacts
+.DS_Store
+.vscode
+.drone.status
+
+# Elixir LS and Tools
+.elixir_ls
+.elixir-tools
+.lexical
+.expert
+
+# AI tools
+.claude
+.codex
+.mcp.json
+
 # The directory Mix will write compiled artifacts to.
 /_build/