refactor(skills): carry the bmad-walkthrough rename into the flat tree

Main renamed bmad-checkpoint-preview to bmad-walkthrough; apply the
same rename under skills/, repoint the hub help doc, and drop the
v6 shim main added since this branch removes the shim tree.

package.json

@@ -34,8 +34,8 @@
     "quality": "npm run format:check && npm run lint && npm run lint:md && npm run docs:build && npm test && npm run validate:refs && npm run validate:skills && npm run docs:validate-sidebar",
     "test": "npm run test:docs && npm run test:python",
     "test:docs": "node docs-site/test/test-site-url.mjs && node docs-site/test/test-rehype-plugins.mjs",
-    "test:python": "PYTHONDONTWRITEBYTECODE=1 uv run --python 3.11 --with pytest --with pytest-xdist --with \"pyyaml>=6.0.2,<7\" --with \"ruamel.yaml>=0.18\" pytest -q -ra --tb=short -n auto -p no:cacheprovider tools/tests skills",
     "test:implementation-model": "node test/test-validate-published-implementation-model.mjs",
+    "test:python": "PYTHONDONTWRITEBYTECODE=1 uv run --python 3.11 --with pytest --with pytest-xdist --with \"pyyaml>=6.0.2,<7\" --with \"ruamel.yaml>=0.18\" pytest -q -ra --tb=short -n auto -p no:cacheprovider tools/tests skills",
     "validate:refs": "uv run --python 3.11 tools/validate_file_refs.py --strict",
     "validate:skills": "uv run --python 3.11 tools/validate_skills.py --strict"
   },

skills/bmad/references/help.md

@@ -45,7 +45,7 @@ Implementation and quality:
   loop dispatches. Do not choose it for attended work.
 - `bmad-code-review` — optional extra review of any change, on top of
   Build's built-in review.
-- `bmad-checkpoint-preview` — guided human walkthrough of a change.
+- `bmad-walkthrough` — guided human walkthrough of a change.
 - `bmad-qa-generate-e2e-tests` — generates API and end-to-end tests for
   implemented code.
 - `bmad-retrospective` — judges a completed epic as a whole against its spec.
@@ -126,7 +126,7 @@ this does not prove completion. Then:
   `bmad-sprint-planning`'s status action.
 - After a Build: `bmad-code-review` is an optional extra gate; offer
   `bmad-qa-generate-e2e-tests` when automated coverage is wanted and
-  `bmad-checkpoint-preview` when a human wants a walkthrough. Recommend
+  `bmad-walkthrough` when a human wants a walkthrough. Recommend
   repeated `bmad-code-review` after material fixes until remaining findings
   no longer affect acceptance. Keep in mind that both build skills have a
   review step, and each `bmad-code-review` run can take up to half an hour or

src/bmm-skills/v6-shims/bmad-checkpoint-preview/SKILL.md

@@ -1,21 +0,0 @@
----
-name: bmad-checkpoint-preview
-description: "Deprecated: forwards to bmad-walkthrough. Do not use unless invoked by name"
-metadata:
-  lifecycle: shim
----
-
-# Deprecated Walkthrough Alias
-
-## On Activation
-
-1. Check whether either legacy customization file exists:
-   - `{project-root}/_bmad/custom/bmad-checkpoint-preview.toml`
-   - `{project-root}/_bmad/custom/bmad-checkpoint-preview.user.toml`
-2. If neither legacy file exists, output exactly `bmad-checkpoint-preview is deprecated. Redirecting to bmad-walkthrough. Please use bmad-walkthrough in the future.`, invoke `bmad-walkthrough` exactly once with the user's original input verbatim, then execute no further steps in this shim.
-3. For every legacy file that exists, use its matching new filename:
-   - `{project-root}/_bmad/custom/bmad-checkpoint-preview.toml` becomes `{project-root}/_bmad/custom/bmad-walkthrough.toml`.
-   - `{project-root}/_bmad/custom/bmad-checkpoint-preview.user.toml` becomes `{project-root}/_bmad/custom/bmad-walkthrough.user.toml`.
-4. If the matching new file does not exist, tell the user that the customization file uses the deprecated name and offer to rename it. Rename it only after explicit approval. If approval is declined or unavailable, or the rename fails, HALT and do not invoke any skill.
-5. If the matching new file already exists, do not overwrite it. Read both files, explain their differences, and propose the exact content for the new file. Resolve conflicting values with the user. Only after the user explicitly approves that content, save and verify the new file, then remove the legacy file. If approval is declined or unavailable, or any operation fails, HALT and do not invoke any skill.
-6. After every detected legacy file has been migrated successfully and no legacy file remains, output exactly `bmad-checkpoint-preview is deprecated. Redirecting to bmad-walkthrough. Please use bmad-walkthrough in the future.`, invoke `bmad-walkthrough` exactly once with the user's original input verbatim, then execute no further steps in this shim.