feat(code-review): integrate fallow structural pre-pass for /gsd-code-review (#3424)

* feat(code-review): add optional fallow structural pre-pass

* fix(ci): sync lockfile for fallow optional binaries

* fix(test): make fallow integration tests cross-platform

* fix(review): require executable fallow binary paths

* docs(review): clarify structural findings usage and size guard

* fix(fallow): preserve line:0, prefer node_modules/.bin, sync SDK twin (H1, M2, N1 from #3424 review)

* fix(workflow): harden fallow pre-pass — exit check, timeout, atomic write, size-guard order (B1, H2-H4, M1, M3 from #3424 review)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix(deps): pin fallow floor to ^2.70.0 matching lockfile (H7 from #3424 review)

* fix(config): enum-validate fallow.scope/profile + group code_quality.* contiguously (H5, N3 from #3424 review)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* docs(fallow): label mcp gate reserved, version-pin install, expand context schema (B3, H8, M4, M8, L1 from #3424 review)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* test(fallow): replace source-grep with behavioral tests, expand fixtures, fail-loud tmpdir (B4, H6, L2, L3, M5, M6, N2 from #3424 review)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix(workflow): escape closing structural_findings tag in JSON payload (CR #3424 inline finding)

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

README.md

@@ -194,6 +194,8 @@ Key dials:
 | `workflow.research` / `plan_check` / `verifier` | Toggle the quality agents that add tokens and time |
 | `parallelization.enabled` | Run independent plans simultaneously |
 
+Optional structural review: set `code_quality.fallow.enabled` to `true` to add a fallow pre-pass to `/gsd-code-review`. GSD writes `.planning/phases/<phase>/FALLOW.json` and surfaces a `Structural Findings (fallow)` section in `REVIEW.md`. Install with `npm install -D fallow@^2.70.0` (or system-wide via `cargo install fallow`; note that the Rust binary's JSON schema must match the documented v2.70+ contract — older versions may produce silent zero-finding output).
+
 For the full configuration reference — all settings, git branching strategies, per-runtime model overrides, workstream config inheritance, agent skills injection — see **[docs/CONFIGURATION.md](docs/CONFIGURATION.md)**.
 
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