chore: add REPO_POLICIES compliance files

Add .gitignore, .editorconfig, REPO_POLICIES.md, and .dockerignore
to bring the repository into compliance with REPO_POLICIES standards.

- .gitignore: Standard template from sneak/prompts plus Go-specific entries
- .editorconfig: root=true, UTF-8, LF, tabs for Go/Makefile, 2 spaces elsewhere
- REPO_POLICIES.md: Copied from sneak/prompts (last_modified: 2026-02-22)
- .dockerignore: Excludes .git, bin/, editor configs, docs; keeps all build files

.editorconfig

@@ -0,0 +1,15 @@
+root = true
+
+[*]
+charset = utf-8
+end_of_line = lf
+insert_final_newline = true
+trim_trailing_whitespace = true
+indent_style = space
+indent_size = 2
+
+[*.go]
+indent_style = tab
+
+[Makefile]
+indent_style = tab