Add transcript description prompt context

docs/architecture.md

@@ -221,6 +221,7 @@ Implemented config surfaces include:
 - module list
 - primary and validation LLM settings
 - total/proposal/validation LLM concurrency controls
+- transcript description context (`--transcript-description`)
 - section token controls and target sections
 - confidence thresholds
 - normalization controls
@@ -229,6 +230,25 @@ Implemented config surfaces include:
 Current caveat:
 - LLM/module-related settings are active for default and explicit module-run paths.
 
+Transcript description behavior:
+- `--transcript-description` is a process-flag input for optional user-supplied background context.
+- runtime config stores this value in `Config.TranscriptDescription` after CLI trimming and length validation.
+- default value is empty; empty values produce no prompt context section.
+- this value is intentionally non-secret and appears in effective config and invocation metadata artifacts.
+
+## Implemented transcript description prompt context
+Transcript description context is wired through production prompt paths:
+- proposal prompts for `glossary`, `homophones`, `spoken_word`, and `grammar`;
+- LLM-backed validator prompts for spoken-form plausibility, meaning reversal, editorial review, grammar review, and spoken-word review.
+
+Prompt guardrail semantics are consistent across modules and validators:
+- transcript description is labeled as "background context only";
+- it may help interpret ambiguous terms;
+- it must not override transcript content;
+- the model must not invent corrections, facts, names, events, motivations, or speaker intent from this description.
+
+Generated transcript descriptions remain deferred and are not implemented in the current runtime.
+
 ## Implemented structured LLM infrastructure
 `internal/framework/contracts` now defines a typed structured-completion contract:
 - `StructuredLLMClient.CompleteStructured(ctx, req, out)`