Telemetry: Local infrastructure & configuration

[nikomatsakis]

Basic plumbing: generate, serialize, and manage telemetry events locally.

Part of the telemetry initiative — see sync discussions: June 9, June 23.

Milestones

• Telemetry event helpers — Define a TelemetryEvent enum/struct (timestamped, tagged by kind). Serialize events as JSON lines into ~/.symposium/telemetry/. Drop events older than 30 days.
• Opt-in during cargo agents init — Add a prompt: "Would you like to enable anonymous telemetry?" Store preference in ~/.symposium/config.toml (e.g. [telemetry] enabled = true). Respect the preference everywhere events are emitted.
• cargo agents telemetry subcommand — status (show whether enabled, where data lives, how much stored), enable/disable (toggle opt-in). Stretch: show to dump recent events for user inspection.

Notes

• Jack has local WIP for basic infra; will open a base PR.
• James waiting on this to land before building on top.
• Only allow enabling telemetry for agents we actually support.