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>

.gitignore

@@ -0,0 +1,72 @@
+# --- Rust ---
+/target/
+**/target/
+Cargo.lock.orig
+*.rlib
+*.rmeta
+
+# Rust keeps its downloaded-crate cache here. It grows to hundreds of MB and is
+# pure cache: delete it any time, cargo re-fetches. Not in git.
+/.cargo-cache/
+
+# --- Android / Gradle ---
+.gradle/
+**/gradle/
+local.properties
+*.apk
+*.aab
+*.keystore
+*.jks
+keystore.properties
+release-keys/
+
+# --- Desktop / release artifacts ---
+/dist/
+*.exe
+*.msi
+*.deb
+*.rpm
+*.AppImage
+
+# --- Secrets — never commit ---
+.env
+.env.*
+api_key.txt
+*.pem
+*.key
+*.p12
+*.pfx
+secrets.toml
+credentials.json
+ssh_key*
+id_ed25519*
+id_rsa*
+
+# --- Android Studio / IntelliJ / VS Code ---
+.idea/
+*.iml
+.kotlin/
+.cxx/
+.vscode/*
+!.vscode/extensions.json
+
+# --- Local databases (never commit runtime state) ---
+*.db
+*.db-journal
+*.db-wal
+*.db-shm
+*.sqlite
+*.sqlite3
+
+# --- OS cruft ---
+.DS_Store
+Thumbs.db
+*~
+*.swp
+
+# --- Local scratch ---
+/tmp/
+*.log
+
+# Claude Code local scheduler state — machine-specific, not project config
+.claude/