phase 1 scaffold: cargo workspace + 3 crates + roadmap + architecture

Repo skeleton for aldabra, the rust-native cardano lite wallet
with MCP server interface. Builds end-to-end, types in place,
real cardano primitives land next pass.

Crates:
  wallet-core   — pure crypto + types. mnemonic, key derivation,
                  signing. No I/O. Security boundary.
  wallet-chain  — pluggable backends. ChainBackend trait, Koios
                  client (stub for now). Ogmios + submit in phase 2.
  wallet-mcp    — the binary. stdio MCP transport via rmcp.

Phase plan in ROADMAP.md, threat model in docs/architecture.md.

This is also Sulkta's first Rust project + a real-world workout for
ci-runner's rust toolchain.

.gitignore

@@ -0,0 +1,18 @@
+# Rust
+target/
+**/*.rs.bk
+Cargo.lock.bk
+
+# Editor
+.idea/
+.vscode/
+*.swp
+*~
+
+# Sulkta — never commit secrets
+*.mnemonic
+*.key
+*.age
+.env
+.env.*
+data/