tui: show non-file layer content in /debug-config

[bolinfest]

The debug output listed non-file-backed layers such as session flags and MDM managed config, but it did not show their values. That made it difficult to explain unexpected effective settings because users could not inspect those layers on disk.

Now /debug-config might include output like this:

Config layer stack (lowest precedence first):
  1. system (/etc/codex/config.toml) (enabled)
  2. user (/Users/mbolin/.codex/config.toml) (enabled)
  3. legacy managed_config.toml (mdm) (enabled)
     MDM value:
       # Production Codex configuration file.

       [otel]
       log_user_prompt = true
       environment = "prod"
       exporter = { otlp-http = {
         endpoint = "https://example.com/otel",
         protocol = "binary"
       }}