chore: add repo hygiene config and project spec

Bootstrap the repository with the shared configuration every other piece
depends on:

- .gitignore: strict. Keeps build output, cargo caches, Android keystores,
  signing keys, .env files, local SQLite databases, IDE state and any
  credential-shaped filename out of history. CLAUDE.md 10.1 requires these
  never enter the repo.
- .editorconfig, rustfmt.toml, clippy.toml: formatting and lint policy.
  clippy.toml is written in the lints-file form so lints are not duplicated
  into every crate manifest.

CLAUDE.md is committed verbatim; it is the project spec and the parity
contract for every platform layer.

Co-Authored-By: Claude Code <noreply@anthropic.com>

.editorconfig

@@ -0,0 +1,19 @@
+# EditorConfig — https://editorconfig.org
+root = true
+
+[*]
+charset = utf-8
+end_of_line = lf
+insert_final_newline = true
+trim_trailing_whitespace = true
+indent_style = space
+indent_size = 4
+
+[*.{yml,yaml,json,toml,md}]
+indent_size = 2
+
+[*.md]
+trim_trailing_whitespace = false
+
+[Makefile]
+indent_style = tab