feat(uv-env): create plugin

This plugin automatically activates/deactivates a uv-managed virtual
environment when you cd into or out of a project directory. It looks
for pyproject.toml and uv.lock to detect a uv project, and reads
UV_PROJECT_ENVIRONMENT to support custom venv locations.

plugins/uv-env/README.md

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+# uv Environment Plugin
+
+This plugin automatically activates the uv virtual environment when you cd into a project directory, and deactivates it when you cd out.
+Note: Script looks for pyproject.toml and uv.lock files to determine if it's a uv project
+
+To use it, add `uv-env` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... uv-env)
+```

plugins/uv-env/uv-env.plugin.zsh

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+_toggleUvShell() {
+  local in_uv_dir=0
+  if [[ -f "$PWD/pyproject.toml" && -f "$PWD/uv.lock" ]]; then
+    in_uv_dir=1
+  fi
+
+  if [[ $uv_active -eq 1 ]] && { [[ $in_uv_dir -eq 0 ]] || [[ "$PWD" != "$uv_dir" && "$PWD" != "$uv_dir"/* ]]; }; then
+    export uv_active=0
+    unset uv_dir
+    (( $+functions[deactivate] )) && deactivate
+  fi
+
+  if [[ $in_uv_dir -eq 1 ]] && [[ $uv_active -ne 1 ]]; then
+    venv_dir="${UV_PROJECT_ENVIRONMENT:-$PWD/.venv}"
+    [[ "$venv_dir" != /* ]] && venv_dir="$PWD/$venv_dir"
+
+    if [[ -f "${venv_dir}/bin/activate" ]]; then
+      export uv_active=1
+      export uv_dir="$PWD"
+      source "${venv_dir}/bin/activate"
+    fi
+  fi
+}
+autoload -U add-zsh-hook
+add-zsh-hook chpwd _toggleUvShell
+_toggleUvShell