docs(test): expand M1 golden-transcript brief tests (mock provider, grounding invariant)

docs/06-test-strategy.md

@@ -114,7 +114,14 @@ is green and the cross-cutting gates pass.
   requires a token, and **opens no outbound socket** — the egress test is unchanged with MCP on (FR-MCP-7,
   NFR-SEC-5). Recordings are not served unless `expose_recordings` is true (FR-MCP-3).
 - Audit: every tool call appends an `mcp_access_log` row / `mcp://access` event (FR-MCP-5).
-- Unit: `FeatureBriefBuilder` produces problem/outcome/acceptance-criteria from a golden transcript.
+- Unit (M1 — feature briefs, no network): (1) `parse_brief` splits a golden
+  `## Title/## Problem/## Desired Outcome/## Acceptance Criteria` reply into the right fields, and an
+  empty/malformed reply yields empty fields without panicking. (2) `FeatureBriefBuilder` over a golden
+  transcript, driven by a `MockLlmProvider` that returns a fixed sectioned reply, produces a
+  schema-valid `FeatureBrief` with non-empty `acceptance_criteria` and satisfies the **grounding
+  invariant**: every `context_excerpts[].text` is a verbatim substring of some transcript segment
+  (never model paraphrase). (3) Command-level: `create_feature_brief` with the LLM off/unreachable
+  returns `Err` and writes no `briefs/*.json` and no `feature_briefs` row (no partial artifacts).
 - (10c, when built) push: `AgentRunner` invokes a stub CLI with the brief; `IssueTracker` creates a
   mocked GitHub issue and (optional) Copilot assignment (FR-AGENT-1/2).