docs: correct MCP error and capture import guidance

docs/process-capture.md

@@ -157,8 +157,9 @@ results for unaffected dimensions and mark affected dimensions unknown; an
 incomplete capture cannot locate the first divergence across all dimensions.
 Trace assertions use coverage for the sources they select, so an assertion
 about complete raw terminal output need not fail because rendered snapshots
-were omitted. Older truncated captures without these details retain the
-conservative comparison behavior. These diagnostics use the existing scenario
+were omitted. Captures missing `truncation_details` are rejected on import;
+preserve them as historical files and recapture for current comparisons.
+These diagnostics use the existing scenario
 budgets; they do not introduce a separate file-hashing budget.
 
 When the host withholds an unrelated process’s ownership token, REA leaves that