docs: clean up ADR-005 architecture docs after envelope implementation

Remove stale ADR-005 drift tables across all architecture docs since
ResponseEnvelope types, factories, detection, and integration points
are now fully implemented in source code. Key changes:

- api-surface.md: Remove ADR-005 drift table (all items implemented),
  retain ADR-006 drift table without execute() return type (now done)
- call-protocol.md: Remove ADR-005 drift table, update ADR-006 table,
  fix CallHandlerConfig to show callMap? (current source)
- adapters.md: Remove 'current source state' and 'implementation
  changes needed' tables for from_mcp and from_openapi, replace with
  current-accurate descriptions of envelope behavior
- response-envelopes.md: Remove 'current source state' blocks,
  update migration checklist to show all code changes completed
- 005-response-envelopes.md: Change status from Draft to Implemented
- 006-unified-invocation-path.md: Update Prerequisites section to note
  ADR-005 is now implemented
- build-distribution.md: Add response-envelope.ts to source layout
- architecture.md: Add response-envelopes.md link and ADR-005/006
  entries to design decisions table
- README.md: Add response-envelopes.md to documents table
- Update last_updated dates on all changed docs

docs/architecture/decisions/005-response-envelopes.md

@@ -1,6 +1,6 @@
 # ADR-005: Response Envelopes for Transport-Aware Results
 
-**Status**: Draft
+**Status**: Implemented
 **Date**: 2026-05-10
 
 ## Context

docs/architecture/decisions/006-unified-invocation-path.md

@@ -1,6 +1,6 @@
 ---
 status: draft
-last_updated: 2026-05-10
+last_updated: 2026-05-11
 ---
 
 # ADR-006: Unified Invocation Path
@@ -31,7 +31,7 @@ Meanwhile `execute()` is a **domain call** — same-process, same-trust, rich ty
 
 ### Prerequisites
 
-This ADR depends on ADR-005 (response envelopes) being implemented in source first. The unified `execute()` requires `ResponseEnvelope` types, `isResponseEnvelope()`, and factory functions that don't exist in source yet.
+This ADR depends on ADR-005 (response envelopes) being implemented in source first. **ADR-005 is now implemented** — `ResponseEnvelope` types, `isResponseEnvelope()`, and factory functions exist in `src/response-envelope.ts`. The prerequisite is met.
 
 ## Decision