docs: add CHANGELOG.md (Keep a Changelog format)

CHANGELOG.md

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+# Changelog — nu-cli
+
+All notable changes to this project will be documented in this file.
+Format: [Keep a Changelog](https://keepachangelog.com/en/1.1.0/)
+
+---
+
+## [Unreleased]
+
+## [0.1.0] — 2026-04-24
+
+### Added
+- `nu wallet new [--label <name>]` — generates secp256k1 keypair, stores private key
+  as hex in `~/.nu/keystore/<label>.key` (plaintext in Faz 0; encrypted in Faz 1)
+- `nu wallet list` — lists all keys in `~/.nu/keystore/`
+- clap CLI scaffold with subcommand structure (`wallet`, `node`, `story` stubs)
+- `src/commands/wallet.rs` — keygen via k256 crate