docs: add a shell completions installation guide

docs/src/SUMMARY.md

@@ -15,6 +15,7 @@
   - [Value Buffer Protocol](reference/value-buffers.md)
   - [Memory & Error Model](reference/memory-error.md)
   - [Naming and Package Conventions](reference/naming.md)
+  - [Shell Completions](reference/completions.md)
 - [Generators](generators/README.md)
   - [Kotlin](generators/kotlin.md)
   - [C](generators/c.md)

docs/src/getting-started.md

@@ -34,6 +34,8 @@ cargo install weaveffi-cli
 
 This puts the `weaveffi` binary on your `PATH`.
 
+For tab completion in your shell, see [Shell Completions](reference/completions.md).
+
 ## 2) Define your API as an IDL
 
 Describe the API once in a language-neutral IDL. Create `math.yml` with a

docs/src/reference/completions.md

@@ -0,0 +1,63 @@
+# Shell Completions
+
+`weaveffi completions <shell>` prints a completion script for bash, zsh, fish, powershell, or elvish. Generate one with:
+
+```bash
+weaveffi completions bash
+```
+
+Replace `bash` with your shell name. Install the script using the matching section below.
+
+## bash
+
+```bash
+mkdir -p ~/.local/share/bash-completion/completions
+weaveffi completions bash > ~/.local/share/bash-completion/completions/weaveffi
+```
+
+On systems that load `/etc/bash_completion.d`:
+
+```bash
+sudo weaveffi completions bash > /etc/bash_completion.d/weaveffi
+```
+
+## zsh
+
+Write the script as `_weaveffi` in a directory on your `$fpath`:
+
+```bash
+mkdir -p ~/.zfunc
+weaveffi completions zsh > ~/.zfunc/_weaveffi
+```
+
+Then in `~/.zshrc` (run `compinit` after updating `fpath`):
+
+```zsh
+fpath=(~/.zfunc $fpath)
+autoload -Uz compinit && compinit
+```
+
+## fish
+
+```bash
+mkdir -p ~/.config/fish/completions
+weaveffi completions fish > ~/.config/fish/completions/weaveffi.fish
+```
+
+## PowerShell
+
+Append the script to your profile:
+
+```powershell
+weaveffi completions powershell >> $PROFILE
+```
+
+Reload the profile or open a new session.
+
+## elvish
+
+Evaluate the script from `rc.elv` (for example):
+
+```elvish
+eval (weaveffi completions elvish | slurp)
+```