docs(readme): add status line wiring snippet to Claude Code section

Claude Code's plugin manifest cannot declare statusLine (only
mcpServers / skills / hooks / lspServers / monitors are auto-wired
on plugin install). Document the one-time manual settings.json edit
inside the Claude Code install section so users see it during
first-time setup, not after they've already moved on.

Snippet uses the path-free `context-mode statusline` form so users
never need to know where Claude Code's plugin cache lives. The bar
mirrors the same value language as ctx_stats:
  \$ saved this session · \$ saved across sessions · % efficient

README.md

@@ -95,6 +95,19 @@ All checks should show `[x]`. The doctor validates runtimes, hooks, FTS5, and pl
 
 > **Note:** Slash commands are a Claude Code plugin feature. On other platforms, type `ctx stats`, `ctx doctor`, `ctx upgrade`, or `ctx insight` in the chat — the model calls the MCP tool automatically. See [Utility Commands](#utility-commands).
 
+**Status line (optional):** Claude Code's plugin manifest cannot declare a status line, so this is a one-time manual edit to `~/.claude/settings.json`:
+
+```json
+{
+  "statusLine": {
+    "type": "command",
+    "command": "context-mode statusline"
+  }
+}
+```
+
+After saving, restart Claude Code. The bar shows `$ saved this session · $ saved across sessions · % efficient` so you can see savings accumulate in real time. The wiring is path-free — `context-mode statusline` resolves through the bundled CLI regardless of where the plugin cache lives.
+
 <details>
 <summary>Alternative — MCP-only install (no hooks or slash commands)</summary>