fix: statusline reads flag from CLAUDE_CONFIG_DIR, not just ~/.claude (#34 follow-up) (#154)

Issue #34 made the hooks honor CLAUDE_CONFIG_DIR when writing the mode flag
($CLAUDE_CONFIG_DIR/.ponytail-active), enforced by tests/hooks.test.js. But
both statusline scripts still hardcoded $HOME/.claude/.ponytail-active, so any
user with CLAUDE_CONFIG_DIR set gets no badge — or a stale mode from a
pre-migration ~/.claude flag that never updates again.

Make both scripts resolve the flag the same way getClaudeDir() does: prefer
CLAUDE_CONFIG_DIR, fall back to ~/.claude. The fallback branch is identical to
the previous behavior, so unset-env users are unaffected. Also corrects the
now-inaccurate path comment in the activation hook header.

hooks/ponytail-statusline.sh

@@ -1,5 +1,6 @@
 #!/usr/bin/env bash
-flag="$HOME/.claude/.ponytail-active"
+# CLAUDE_CONFIG_DIR overrides ~/.claude, matching where the hooks write the flag (issue #34)
+flag="${CLAUDE_CONFIG_DIR:-$HOME/.claude}/.ponytail-active"
 [ -f "$flag" ] || exit 0
 
 mode=$(head -n1 "$flag" | tr -d '[:space:]')