Tighten Wails release design after self-review

- Fold action.yml into a single composite step so the binary's
  resolved path is computed in the same shell that just installed it.
- Specify shell-style splitting for extra-build-flags.
- Spell out auto-detection edge cases for notarization-method.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/superpowers/specs/2026-05-02-wails-release-action-design.md

@@ -105,9 +105,6 @@ outputs:
 runs:
   using: composite
   steps:
-    - shell: bash
-      run: |
-        go install "${{ github.action_repository }}/cmd/wails-release@${{ github.action_ref }}"
     - shell: bash
       env:
         INPUT_WORKING_DIRECTORY:           ${{ inputs.working-directory }}
@@ -128,9 +125,15 @@ runs:
         INPUT_S3_KEY:                      ${{ inputs.s3-key }}
         INPUT_S3_ENDPOINT_URL:             ${{ inputs.s3-endpoint-url }}
         INPUT_S3_REGION:                   ${{ inputs.s3-region }}
-      run: wails-release
+      run: |
+        go install "${{ github.action_repository }}/cmd/wails-release@${{ github.action_ref }}"
+        "$(go env GOPATH)/bin/wails-release"
 ```
 
+The install and the binary invocation are kept in a single step so we
+don't depend on `$GOPATH/bin` being on `PATH` between steps. The binary
+is invoked by absolute path resolved via `go env GOPATH`.
+
 ### Boundaries
 
 Each `internal/*` package owns one phase, exposes a small interface, and
@@ -170,12 +173,30 @@ work as designed.
 ### Validation rules (config package, before any side effects)
 
 - Both cert inputs must be present.
-- Notarization: exactly one credential group must be complete; mixed or
+- Notarization:
-  partial groups → fail fast with a message naming the missing fields.
+  - If `notarization-method` is `api-key` or `apple-id`, the named
+    group's fields must all be present; missing fields → error listing
+    them by name.
+  - If `notarization-method` is empty or `auto`, exactly one group must
+    be **fully** populated. Both populated → ambiguity error. Neither
+    populated → missing-credentials error.
 - If `s3-bucket` is set, `s3-key` must also be set.
 - Resolve `version` early so it is available for `s3-key` substitution and
   the artifact filename.
 
+### `extra-build-flags` parsing
+
+The value is split with shell-style word rules (Go's
+`github.com/google/shlex` or equivalent) so the workflow can write
+quoted arguments naturally:
+
+```yaml
+extra-build-flags: -tags release -ldflags "-X main.commit=$SHA"
+```
+
+The split tokens are appended to the `wails build` argv after the
+action's mandatory flags (`-platform darwin/universal -clean -trimpath`).
+
 ## Outputs
 
 | Name | Description |