feat: post-merge build & test gate — Build step, iOS/Xcode, serial mode (#2751)

* feat: post-merge build & test gate — Build step, iOS/Xcode, serial mode

Step 5.6 of execute-phase is extended per #2720:
- Renamed from "Post-merge test gate (parallel mode only)" to "Post-merge build & test gate"
- Gate now runs in both parallel mode (after worktree merge) and serial mode (after last plan)
- Added Step A: Build gate resolving BUILD_CMD from workflow.build_command config key, then auto-detecting via priority: config override → Xcode (.xcodeproj) → Makefile build: → Justfile → Cargo/Go/Python/npm. Xcode uses xcodebuild -list -json to get first scheme, then xcodebuild build -scheme ... -destination 'platform=iOS Simulator,name=iPhone 16'. Build failure increments WAVE_FAILURE_COUNT.
- Added Xcode/iOS detection to Step B (Test gate): when *.xcodeproj present and no workflow.test_command configured, uses xcodebuild test instead of the previous "no test runner detected" skip. Scheme reused from Step A when available.
- Documented workflow.build_command and workflow.test_command in docs/CONFIGURATION.md (table + JSON schema)

Closes #2720

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

* refactor(execute-phase): extract Step 5.6 body to post-merge-gate.md sub-file

Moves the build-detection logic and xcodebuild commands from the inline
Step 5.6 body into execute-phase/steps/post-merge-gate.md, replacing it
with a single Read() reference. Reduces execute-phase.md from 1755 to
1647 lines, satisfying the ≤1700 XL-tier budget enforced by
tests/workflow-size-budget.test.cjs.

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

---------

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

docs/CONFIGURATION.md

@@ -53,7 +53,9 @@ GSD stores project settings in `.planning/config.json`. Created during `/gsd-new
     "security_enforcement": true,
     "security_asvs_level": 1,
     "security_block_on": "high",
-    "post_planning_gaps": true
+    "post_planning_gaps": true,
+    "build_command": null,
+    "test_command": null
   },
   "hooks": {
     "context_warnings": true,
@@ -209,6 +211,8 @@ All workflow toggles follow the **absent = enabled** pattern. If a key is missin
 | `workflow.inline_plan_threshold` | number | `3` | Maximum number of tasks in a phase before the planner generates a separate PLAN.md file instead of inlining tasks in the prompt |
 | `workflow.drift_threshold` | number | `3` | Minimum number of new structural elements (new directories, barrel exports, migrations, route modules) introduced during a phase before the post-execute codebase-drift gate takes action. See [#2003](https://github.com/gsd-build/get-shit-done/issues/2003). Added in v1.39 |
 | `workflow.drift_action` | string | `warn` | What to do when `workflow.drift_threshold` is exceeded after `/gsd-execute-phase`. `warn` prints a message suggesting `/gsd-map-codebase --paths …`; `auto-remap` spawns `gsd-codebase-mapper` scoped to the affected paths. Added in v1.39 |
+| `workflow.build_command` | string | (none) | Shell command to build the project in the post-merge build gate (Step A of step 5.6 in execute-phase). When unset, the gate auto-detects: Xcode (`.xcodeproj` present) → `xcodebuild build`, `Makefile` with `build:` target → `make build`, Justfile → `just build`, `Cargo.toml` → `cargo build`, `go.mod` → `go build ./...`, Python → `python -m py_compile`, `package.json` with `build` script → `npm run build`. Runs with a 5-minute timeout; failure increments `WAVE_FAILURE_COUNT`. Added in v1.39 |
+| `workflow.test_command` | string | (none) | Shell command to run the project's test suite in the post-merge test gate (Step B of step 5.6 in execute-phase) and the regression gate. When unset, the gate auto-detects: Xcode (`.xcodeproj` present) → `xcodebuild test`, `Makefile` with `test:` target → `make test`, Justfile → `just test`, `package.json` → `npm test`, `Cargo.toml` → `cargo test`, `go.mod` → `go test ./...`, Python → `python -m pytest`. Runs with a 5-minute timeout; failure increments `WAVE_FAILURE_COUNT`. Added in v1.39 |
 
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