docs: SonnetPlan hack — budget Sonnet+Haiku hybrid via env var remap

- guide/ultimate-guide.md §OpusPlan: new "Budget Variant: SonnetPlan"
  section with shell function, Plan/Act routing table, caveat on
  unreliable model self-report, link to GitHub issue #9749
- examples/scripts/sonnetplan.sh: new ready-to-use shell function
  with installation instructions and verification guidance

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

examples/scripts/sonnetplan.sh

@@ -0,0 +1,32 @@
+#!/usr/bin/env bash
+# sonnetplan — Budget hybrid mode: Sonnet 4.6 (plan) + Haiku 4.5 (act)
+#
+# How it works:
+#   Claude Code's `opusplan` alias routes to Opus (plan) + Sonnet (act).
+#   By remapping the environment variables that resolve the `opus` and `sonnet`
+#   aliases, we effectively create a cheaper Sonnet→Haiku hybrid.
+#
+# Usage:
+#   sonnetplan                    # Start Claude in current directory
+#   sonnetplan --project /path    # Start in specific project
+#
+# Installation:
+#   Add this function to your ~/.zshrc or ~/.bashrc:
+#
+#     source /path/to/sonnetplan.sh
+#
+#   Or copy the function body directly into your shell config.
+#
+# Verification:
+#   After running `sonnetplan` and typing `/model opusplan`, check the
+#   status bar — it should show "Model: Sonnet 4.6" in plan mode.
+#   Do NOT rely on asking the model "what model are you?" — that self-report
+#   is unreliable. Use the status bar or your billing dashboard instead.
+#
+# GitHub issue tracking native support: https://github.com/anthropics/claude-code/issues/9749
+
+sonnetplan() {
+    ANTHROPIC_DEFAULT_OPUS_MODEL=claude-sonnet-4-6 \
+    ANTHROPIC_DEFAULT_SONNET_MODEL=claude-haiku-4-5-20251001 \
+    claude "$@"
+}