docs(release): add v0.4.0 release notes header

.github/releases/v0.4.0.md

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+## `things-cli` v0.4.0 — shell completions
+
+`things <TAB>` now completes subcommands, flags, and enum values in bash, zsh,
+and fish. Homebrew sets it up for you; everyone else runs one line.
+
+### Headline
+
+- **Shell completions** for bash, zsh, and fish (#94, #95). The Homebrew cask
+  generates them on install, so `brew install ryanlewis/tap/things` gives you
+  `things <TAB>` with no extra steps. On other install paths, load them
+  yourself — `source <(things completions bash)`, `source <(things completions
+  zsh)`, or `things completions fish | source`. Completions are driven from the
+  live command tree, so they never drift from the CLI: subcommand names, flag
+  names, and enum values like `--color auto|always|never`.
+
+### Under the hood
+
+- Migrated styling to [lipgloss v2](https://github.com/charmbracelet/lipgloss)
+  (`charm.land/lipgloss/v2`) (#93). No visible change to `list`/`show` output;
+  JSON (`-j`) stays script-stable.
+- Routine dependency updates (#83, #91, #92).
+
+### Docs
+
+- The bundled agent skill (`things skill show` / installed `SKILL.md`) now spells
+  out every `things open` flag (`-p`/`-a`/`-t`/`-q`/`--filter`/`--background`)
+  and the `edit` / `project edit` prepend/append/checklist/`*-id` flags instead
+  of trailing `...` (#89). Reinstall it (`things skill install claude`, `…codex`,
+  `…pi`) to pick up the new text.
+
+### Requirements
+
+macOS with Things3 installed. Binaries for Apple Silicon (`darwin_arm64`) and
+Intel (`darwin_amd64`).