[FEATURE] Display telemetry notification and destination on startup

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

Currently, when OpenTelemetry telemetry is enabled (especially via managed-settings), Claude Code silently begins transmitting data without user awareness. This raises security and privacy concerns:

1. Users may not know their usage data is being collected and transmitted
2. Data includes personal information (email addresses, user IDs, session IDs, organization IDs)
3. When OTEL_LOG_USER_PROMPTS=1 is set, prompt content is also logged
4. In enterprise environments, telemetry may be configured via managed-settings without individual users' explicit knowledge or consent
5. Data is sent to third-party services (Datadog, Grafana, Honeycomb, etc.) that users have no direct contractual relationship with

This lack of transparency is a security concern and doesn't align with best practices for informed consent.

Proposed Solution

On startup, when telemetry is enabled (CLAUDE_CODE_ENABLE_TELEMETRY=1), display a notification showing:

• That telemetry is active
• The destination endpoint (OTEL_EXPORTER_OTLP_ENDPOINT value)
• What types of data are being collected (metrics, logs, and whether prompts are included)
• Option to proceed or disable for current session

Example output:

⚠️  OpenTelemetry Telemetry Active
   Endpoint: https://company-monitoring.example.com:4317
   Collecting: Metrics, Logs
   Prompt logging: Enabled (OTEL_LOG_USER_PROMPTS=1)
   
   Press Enter to continue or Ctrl+C to exit

This ensures users are aware of data transmission before it begins, similar to how browsers and other IDEs handle telemetry.

Alternative Solutions

1. Display notification only on first startup (less secure but less intrusive)
2. Add a /telemetry status command to check current configuration (reactive rather than proactive)
3. Log telemetry information to a file instead of interactive prompt (less visible to users)
4. Add a configuration option to suppress the notification (allows informed users to opt out of the prompt)

Priority

Medium - Would be very helpful

Feature Category

Configuration and settings

Use Case Example

Scenario: An employee at a company starts Claude Code, unaware that their IT department has configured managed-settings to send all telemetry (including prompts) to a company monitoring server.

Current behavior: Claude Code starts silently, and all usage data begins transmitting to the company server without the employee's knowledge.

Proposed behavior: On startup, Claude Code displays:

⚠️  OpenTelemetry Telemetry Active
   Endpoint: https://monitoring.company.com:4317
   Collecting: Metrics, Logs, Prompts
   
   Press Enter to continue or Ctrl+C to exit

The employee is now aware they're being monitored and can make informed decisions about what they work on in Claude Code.

Additional Context

• Related to Issue #5508 which addresses inability to disable telemetry, but this is a separate concern about user awareness even when telemetry is intentionally enabled
• Similar transparency features exist in VS Code, Docker Desktop, and major browsers
• This aligns with employment law in many jurisdictions requiring employee notification of monitoring