fix: Add telemetry opt-out via installer prompt and env var

Sentry error reporting can now be disabled by:
1. Declining during the interactive installer (sets CODEGRAPH_TELEMETRY=off
   in the MCP server config env)
2. Setting CODEGRAPH_TELEMETRY=off in your shell environment

README updated with a Telemetry section documenting what is collected
and how to opt out.

Closes #68

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

README.md

@@ -165,6 +165,7 @@ The interactive installer will:
 - Prompt to install `codegraph` globally (needed for hooks & MCP server to work)
 - Configure the MCP server in `~/.claude.json`
 - Set up auto-allow permissions for CodeGraph tools
+- Ask about [anonymous error reporting](#-telemetry) (opt-out available)
 - Add global instructions to `~/.claude/CLAUDE.md` (teaches Claude how to use CodeGraph)
 - Install Claude Code hooks for automatic index syncing
 - Optionally initialize your current project
@@ -309,11 +310,12 @@ npx @colbymchenry/codegraph       # Run via npx (no global install needed)
 The installer will:
 1. Prompt to install `codegraph` globally (needed for hooks & MCP server)
 2. Ask for installation location (global `~/.claude` or local `./.claude`)
-3. Configure the MCP server in `claude.json`
-4. Optionally set up auto-allow permissions
-5. Add global instructions to `~/.claude/CLAUDE.md` (teaches Claude how to use CodeGraph)
-6. Install Claude Code hooks for automatic index syncing
-7. For local installs: initialize and index the current project
+3. Optionally set up auto-allow permissions
+4. Ask about [anonymous error reporting](#-telemetry) (opt-out available)
+5. Configure the MCP server in `claude.json`
+6. Add global instructions to `~/.claude/CLAUDE.md` (teaches Claude how to use CodeGraph)
+7. Install Claude Code hooks for automatic index syncing
+8. For local installs: initialize and index the current project
 
 ### `codegraph init [path]`
 
@@ -709,6 +711,25 @@ Run `codegraph init` in your project directory first.
 - Check if the file's language is supported
 - Verify the file isn't excluded by config patterns
 
+## 📡 Telemetry
+
+CodeGraph collects anonymous error reports via [Sentry](https://sentry.io) to help diagnose and fix bugs. This is **enabled by default** in production environments (disabled in development/test).
+
+**What is collected:**
+- Error type, message, and stack trace (includes local file paths in the trace)
+- CodeGraph version and process name (CLI or MCP server)
+
+**What is NOT collected:**
+- Source code contents
+- File contents or repository data
+- Personal information or environment variables
+
+**To opt out**, set the environment variable before running CodeGraph:
+
+```bash
+export CODEGRAPH_TELEMETRY=off
+```
+
 ---
 
 ## 📄 License