Preserve trace file read causes - #3300

I reviewed the changed in-scope file apps/server/src/diagnostics/TraceDiagnostics.ts (and its test) against the Effect service conventions. The changes are compliant:

• Imports: All Effect modules are imported as namespaces from their subpaths (import * as Result from "effect/Result", import * as Schema from "effect/Schema", and in the test import * as Logger from "effect/Logger", import * as References from "effect/References"). @t3tools/contracts correctly retains named type imports.
• Error modeling: The new TraceFileReadError uses Schema.TaggedErrorClass with structured attributes, derives its message exclusively from the stable traceFilePath attribute (not from cause/cause.message), and preserves the immediate underlying PlatformError as a required cause (Schema.Defect()). This matches the recommended pattern where a semantically precise error tag preserves a real cause.
• Diagnostic field: causeTag is a multi-value, serializable normalized category (cause.reason._tag) used only for log annotations/diagnostics, not to select the message or drive control flow, so it is permitted to remain a field.
• Service layout/shape: Canonical order (imports → error declaration → Context.Service tag with inline interface → make → layer) is preserved, and the service interface is referenced via TraceDiagnostics["Service"]["read"].
• Behavior/tests: The message now derives from structural attributes rather than stringifying the platform error, which aligns with the error conventions, and the existing test was updated (not bloated) to match.

No convention violations were introduced or retained in the changed scope.

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Agent Credits: 28 credits