Telemetry: document local telemetry behavior and privacy guidance

[EhabY]

Follow-up from the local telemetry audit of #953 / #903.

Problem

Before we encourage users to export/share local telemetry, we should document what is collected, where it lives, and the basic privacy/cardinality rules future instrumentation should follow.

This issue is intentionally documentation/guidance only. Implementation work for export hardening and new instrumentation is tracked separately in #983, #984, #985, and #986.

Scope

User-facing documentation

Document:

• what coder.telemetry.level = local means
• where local telemetry files are stored
• retention/rotation behavior
• what kinds of data may appear in exports
• how to disable local telemetry
• how to export/review telemetry before sharing
• how telemetry relates to support bundles once Support bundles should include recent local telemetry #952 lands

Lightweight contributor guidance

Add a short telemetry guidance section for future instrumentation:

• use bounded event names and properties
• avoid raw query strings, file paths, tokens, task prompts/messages, and arbitrary user-provided content
• prefer enums/buckets over free-form error strings or high-cardinality dimensions
• be deliberate about deployment URLs, workspace names, agent names, machine IDs, and session IDs
• keep metrics low-cardinality

Out of scope

• local telemetry management commands such as clear/open/show disk usage
• Chrome Trace/Perfetto timeline export
• a full schema registry or static validation framework
• support-bundle telemetry inclusion, which is already tracked in Support bundles should include recent local telemetry #952
• export hardening work, which is tracked in Telemetry: harden local export activation, privacy, and OTLP output #983
• additional instrumentation, which is tracked in Telemetry: instrument auth, deployment, and credential lifecycle #984, Telemetry: instrument CLI resolution and remote setup phases #985, and Telemetry: instrument workspace and diagnostic command workflows #986

Acceptance criteria

• Users can understand what local telemetry is, where it is stored, how long it is retained, how to disable it, and what exports may contain.
• Contributors have concise privacy/cardinality guidance for adding telemetry events.
• The docs link to the related implementation issues where appropriate.

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Updated by Coder Agent from the telemetry audit of #953.