Follow-up: expose Trace seam for config-loader diagnostics (dime T2) #245

Origin

Dime Ultrareview cycle-1 on PR #241 (fix/device-validation-level-hardening), tracked finding T2 — surfaced by the code-review agent.

Finding

PR #241 replaces the four silent `catch { }` blocks in `AgentConfiguration.ReadJson` / `ReadJson(Type,…)` / `ReadYaml` / `ReadYaml(Type,…)` with:

1. A rethrow of `ArgumentOutOfRangeException` wrapped as `ArgumentException` with the configuration path attached (bad-enum config surfaces the actionable setter message).
2. A `System.Diagnostics.Trace.TraceError($"Config load failed: {configurationPath}: {ex.Message}")` call before the null-return preservation path.

The Trace call surfaces the diagnostic to any `TraceListener` a host has registered — but that's a global-registry side effect the tests can only observe by installing a listener during `[SetUp]`. A test-friendly `ILogger` / `Action<string, Exception>` seam would let unit tests assert the diagnostic surface without touching the global `Trace` machinery.

Rationale for defer

Adding a logging seam is API-surface-shaped work — it changes the `AgentConfiguration` public shape (or introduces a new `static Action<string, Exception>? OnConfigLoadFailure` static delegate seam). That's a decision the maintainer should weigh independently of the H2 swallow fix; it does not gate landing the swallow removal.

Suggested design (for the follow-up PR)

• Add `public static Action<string, Exception>? OnConfigLoadFailure { get; set; }` on `AgentConfiguration`.
• Invoke it inside the H2 catch fallback, alongside the existing `Trace.TraceError` call.
• Add a unit test that sets the delegate, triggers a malformed-JSON load, and asserts the delegate was invoked with the expected path + exception.
• Document the seam in `docs/reference/configuration.md` under a new "diagnostics" subsection.

Refs: dime cycle-1 T2 (code-review) — see PR #241's `AgentConfiguration.cs` H2 fix.