Add session -p <project> flag

This flag sets the session name to `<project>` where `<project>` is a
git repository to make working with multiple worktrees more streamlined.

session/session.plugin.zsh

@@ -1,12 +1,48 @@
 session() {
   if [[ "$1" == "" ]]; then
-    echo "usage: session [-h] <name> [<host>]"
+    echo "usage: session [-h] <name> [<host>]
+       session -p <project>"
   elif [[ "$1" == "-h" ]] || [[ "$1" == "--help" ]]; then
     echo "usage: session [-h] <name> [<host>]
+       session -p <project>
 
 Create or attach to a tmux session by name either locally on on a remote host
 via ssh.
+
+With -p create or attach to a session for <project>, a path in ~/Projects with
+the prefix removed, which must be a git repository. The session is named after
+the project and holds a home window plus a window at the project, named after
+its default branch.
 "
+  elif [[ "$1" == "-p" ]] || [[ "$1" == "--project" ]]; then
+    local project=${2%/}
+    if [[ "$project" == "" ]]; then
+      echo "$fg[red]error:$reset_color missing <project> argument"
+      return 1
+    fi
+    if [[ "$3" != "" ]]; then
+      echo "$fg[red]error:$reset_color invalid argument: $3"
+      return 1
+    fi
+    local repo=$HOME/Projects/$project
+    if [ ! -d "$repo" ]; then
+      echo "$fg[red]error:$reset_color no such project: $project"
+      return 1
+    fi
+    if ! git -C "$repo" rev-parse --git-dir &>/dev/null; then
+      echo "$fg[red]error:$reset_color not a git repository: $repo"
+      return 1
+    fi
+    # tmux rewrites `.` and `:` in session names, so mirror that to keep the
+    # name we create and the name we target in sync.
+    local name=${project//[.:]/_}
+    if [[ "$TMUX" == "" ]]; then
+      tmux new-session -As "$name" -c $HOME -e "TMUX_PROJECT=$project"
+    else
+      tmux has-session -t "=$name" &> /dev/null || \
+        tmux new-session -Ads "$name" -c $HOME -e "TMUX_PROJECT=$project"
+      tmux switch-client -t "=$name"
+    fi
   else
     local name=$1
     local host=$2