Error and Doc audit: first pass

[zeeshanlakhani]

Summary

We need to do an entire error and doc audit of the codebase, particularly the homestar-runtime.

This includes:

• correcting levels and which tracing field names we want as top-level "keys."
• Adding category and subject fields to all logs.
• Even though the runtime should leverage anyhow, and does, we should designate specific error types to bubble up things throughout the runtime and capture fields we care about.
  - [ ] Use anyhow::context more judiciously for error handling contextual information. -> moved to Errors: Better formatting for anyhow/ contexts and thiserror backtraces #455
  - [ ] Handle backtraces in logs and other formatting concerns for logfmt. -> moved to Errors: Better formatting for anyhow/ contexts and thiserror backtraces #455)
• Correct expect phrases.
• allow_dead_code checks/rechecks
• pub/private/visibility checks