feat: v0.9.5 context in tokens as well as percent

Claude's footer reads "ctx 34% 346k/1M"; the Sessions panel and the
session switcher use the same label via ctxLabel(). Closes #43

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WHRLCCdXYm41Qx5kNhehaP

tests/attach.bats

@@ -195,7 +195,7 @@ claude_args() {
       "cost":{"total_cost_usd":1.5,"total_lines_added":10,"total_lines_removed":2}}'
     run bash -c "echo '$status_json' | TMUX='$DEVDESK_SOCKET,1,0' TMUX_PANE='$pane' '$REPO/libexec/devdesk-statusline'"
     [ "$status" -eq 0 ]
-    [[ $output == *"alpha"* && $output == *"ctx 34%"* ]]
+    [[ $output == *"alpha"* && $output == *"ctx 34% 346k/1M"* ]]
     run devdesk attach --list
     [ "$(jq -r '.sessions[] | select(.name=="alpha") | .context.used_tokens' <<<"$output")" = 346000 ]
     [ "$(jq -r '.sessions[] | select(.name=="alpha") | .context.model' <<<"$output")" = Opus ]