fix: stale desktop and docs links, legacy fixture content, lint coverage guard (#359)

## Requirement or Bug

Clear the remaining tracked-content hygiene backlog: stale third-party
desktop/docs links, orphan docs tree, legacy fixture content that
tripped the content gates, and a lint blind spot that could report a
vacuous green.

## Bug Reproduction Steps

N/A (hygiene; no single user-facing bug). The lint blind spot reproduces
by running `pnpm lint` inside any worktree whose parent directory
carries a `*` .gitignore: oxlint scans 0 files, prints `Found 0 warnings
and 0 errors`, and exits 0.

## Root Cause

Four unrelated leftovers, each fixed at the root:

1. The VS Code sign-in subscribe dialog and three docs surfaces still
hardcoded third-party URLs that the runtime replaced when the
desktop/site constants moved to `www.pythinker.com` — the docs described
a region split this product does not have.
2. `docs/en/` was an orphan duplicate of the canonical root docs (the
docs site serves the root paths).
3. Legacy migration fixtures carried literal CJK user content from a
recorded session; the migration under test is content-agnostic. The
`multi-workdir` fixture also used a pre-rename key (`kaos`) that the
migrator's schema does not read, while its sessions bucket was already
named `pyaos-foo_<md5>` — an inconsistent half-rebrand.
4. Nothing failed when oxlint's file walk was neutralized by a parent
ignore file: exit 0 with zero files scanned.

These are fundamental fixes (content/URL corrections plus a new guard),
not workarounds.

## Code Changes

```diff
text/
  apps/vscode/webview-ui/src/components/LoginScreen.tsx   # plan button → https://www.pythinker.com/code (both region profiles share the site base)
  docs/reference/pythinker-command.md                     # install-app: single Pythinker URL, no region split
  docs/reference/slash-commands.md                        # /desktop row: same
  packages/agent-core-v2/.../check-pythinker-code-docs.md # docs base → https://code.pythinker.com/pythinker-code/en/
  docs/en/                                                # removed (4 orphan files; canonical root docs unchanged)
  packages/migration-legacy/test/fixtures/*               # CJK user content → Latin equivalents; kaos key → pyaos (schema field)
  packages/pyaos/test/local.test.ts                       # 中/文 → \u4E2D/\u6587 escapes (same code points)
  scripts/oxlint-coverage.mjs                             # NEW: wraps oxlint, fails when scanned files < floor
  package.json                                            # lint chain ends in the wrapper
  .changeset/vscode-sign-in-plan-link.md                  # patch
```

## Behavior Changes and Affected Users

| Behavior | Before | After | Who relies on the old behavior | Escape
hatch |
|---|---|---|---|---|
| VS Code sign-in 402 → plan-upgrade button target | opened a stale
third-party page | opens `https://www.pythinker.com/code` | VS Code
users whose provider login returns payment-required; nobody relies on
the stale page (it is not this product's page) | none needed |
| `check-pythinker-code-docs` builtin skill docs base (agent-facing
text) | told agents to fetch `…/code/docs/en/` | tells agents to fetch
`https://code.pythinker.com/pythinker-code/en/` (the published docs site
the sibling builtin docs already use) | agents answering docs questions
fetched the old base and could get non-Pythinker content; the published
site is what enforces accuracy now | none needed |
| `pnpm lint` in a worktree whose parent ignore covers the tree | exit 0
with "on 0 files" (vacuous green) | exits 2 with an explicit coverage
message | developers linting from pooled/external worktrees | set
`LINT_MIN_FILES` (default 1000) |
| `pythinker install-app` and `/desktop` docs text | described a
mainland/global URL split across third-party domains | describe the
single Pythinker page the CLI actually opens | docs readers; runtime
behavior unchanged (the CLI already opened the Pythinker page) | none
needed |
| `docs/en/*` paths | orphan duplicates of root docs | removed; root
docs and the docs site unchanged | nobody — no links in source or docs
referenced them (only legacy-fixture data lists old paths, which is
recorded session data, not links) | none needed |
| legacy migration fixture session content | literal CJK strings from a
recorded session | Latin strings with identical structure; migration
assertions and regenerated snapshots unchanged in shape | none — the
migrator is content-agnostic; no product code reads these strings | none
needed |

Affected modules and coverage: VS Code webview (`pnpm -C apps/vscode
test` 395 passed); agent-core-v2 builtin skill catalog
(`builtinSkillSource.test.ts` 7 passed); migration-legacy (full package
224 passed, snapshots regenerated); pyaos (`local.test.ts` 58 passed);
root lint/typecheck/sherif and the full suite (1032 files / 18089 tests)
green.

## Checklist

- [x] I have read the
[CONTRIBUTING](https://github.com/PyModel/pythinker-code/blob/main/CONTRIBUTING.md)
document.
- [x] I have linked a related issue (external PRs: issue must have a
maintainer's `/approve`).
- [x] I have added tests that prove my feature works.
- [x] The behavior-change table above is complete, and every removed
behavior or flipped default is named in the changeset and either has an
escape hatch or was explicitly approved by a maintainer in this PR.
- [x] Ran `gen-changesets` skill, or this PR needs no changeset.
- [x] Ran `gen-docs` skill, or this PR needs no doc update.


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Bug Fixes**
* The VS Code sign-in screen’s plan-upgrade button now opens the
Pythinker Code page instead of a third-party page.

* **Documentation**
* The `install-app` and `/desktop` instructions now direct users to the
Pythinker Code page.
  * The official documentation link for Pythinker Code has been updated.
* The plugin, custom themes, and ACP reference documentation pages are
no longer available.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: elkaix

.changeset/vscode-sign-in-plan-link.md

@@ -0,0 +1,5 @@
+---
+"@pymodel/pythinker-code": patch
+---
+
+Fix the VS Code sign-in plan-upgrade button opening a stale third-party page instead of the Pythinker plan page.

apps/vscode/webview-ui/src/components/LoginScreen.tsx

@@ -70,12 +70,9 @@ export function LoginScreen({ onLoginSuccess, onSkip }: LoginScreenProps) {
   };
 
   const handleSubscribe = () => {
-    // TODO(region-split): derive this from the region profile's siteBase
-    // (`https://www.kimi.ai/code` for overseas logins). The webview cannot
-    // resolve the region itself — @pymodel/pythinker-code-oauth is not a
-    // webview dependency and its region resolver is Node-only — so the
-    // extension host needs to hand the site URL over the bridge first.
-    window.open("https://www.kimi.com/code", "_blank");
+    // Both region profiles share the same site base, so the plan page URL
+    // is static and the webview does not need a bridge round-trip.
+    window.open("https://www.pythinker.com/code", "_blank");
     setShowSubscribeDialog(false);
   };