feat(sdk): add dsh-plugin-fetch source + fetcher seam

Greenfield #2 "建插件" modules for the forthcoming `dsh-sdk create <source>`
command, in a new foundation-independent package so it touches none of the
dsh-scripts / dsh-helper / create-sdk hotspots the foundation refactor edits.

- `resolvePluginSource(spec)` parses `owner/repo[/subdir]#ref` (github) or
  `pkg@version` (npm) into a `PluginSource` discriminated union, failing loud on
  an ambiguous or malformed spec.
- `PluginFetcher<S>` seam + `fetchPlugin` tag dispatch returning a common
  `FetchedPlugin` (temp dir + immutable provenance).
- `GigetFetcher` (github) over @bluwy/giget-core: resolve `#ref` to a commit SHA
  first, download that SHA; provenance pins the SHA. Chosen over unjs/giget for
  its single runtime dep and absent install/action surface.
- `PacoteFetcher` (npm) over pacote: resolve the manifest, then extract the
  tarball verified against its registry integrity. Registry-only is enforced by
  the source resolver; extract runs no lifecycle scripts.
- Branded `CommitSha`/`Integrity`; network + temp-dir boundaries are injected so
  the logic is unit-tested at 100% per-file coverage without network.

Wiring (package.json pin, cordis.yml via ProjectEditSession with a confirmed
diff, install --ignore-scripts) and the launcher command registration land
later with the foundation.

scripts/verify-package-readme-model-experience.ts

@@ -52,6 +52,7 @@ const SENTENCE_MODEL_EXPERIENCE: Readonly<Record<string, SentenceContract>> = {
   'packages/sandbox/sandbox-local': { kind: 'indirect', reason: 'The provider backend delegates model rendering to dsh-bash-sandbox and dsh-tool-bash.' },
   'packages/sdk/create-sdk': { kind: 'indirect', reason: 'The initializer only writes project files; selected runtime plugins provide the generated project model surface.' },
   'packages/sdk/helper': { kind: 'none', reason: 'The project domain edits files and registers no live agent or model surface.' },
+  'packages/sdk/plugin-fetch': { kind: 'none', reason: 'The fetcher acquires plugin sources into a temp dir and registers no live agent or model surface.' },
   'packages/sdk/scripts': { kind: 'indirect', reason: 'The launcher delegates model context to the loaded project plugin tree.' },
   'packages/session-query/session-query': { kind: 'none', reason: 'The trusted query service exposes cloned records only to callers and registers no model surface.' },
   'packages/skill/skill': { kind: 'indirect', reason: 'The provider registry delegates model rendering to dsh-tool-skill.' },