fix(review): validate skill roots at mount and isolate provider default roots

ds-review-bot round 1 on the repository-plugin runtime:
- a manifest-declared skill root absent or non-directory in the installed
  package now fails the plugin load (skill-local treats a missing root as
  legitimately empty, which silently mounted a skill-less plugin)
- includeDefaultRoots: false no longer inherits $DSH_BUNDLED_SKILL_DIR, so
  isolated repository providers see only their explicit roots
- prepared wrapper baseUrl schema requires the file: scheme, failing hostile
  URLs at the declared validation boundary
- preparedPath reuses format.ts's isOutside; SERVER_NAME_PATTERN is exported
  and pinned equal to dsh-mcp-client's, with the restatement justified (the
  prepare bin keeps a zod-only module graph); the unexplained `as never`
  cast now carries its schemastery rationale
- the import-free wrapper assertion also rejects dynamic import(
- the headless fixture wrapper is regenerated by the real prepareDshPlugin
  and a drift test pins fixture == generator output
- prepareDshPlugin JSDoc states the non-atomic publish repair contract

packages/cordis/repository-plugin/README.md

@@ -36,7 +36,7 @@ The containing package manager still runs the configured repository package's li
 
 ## Runtime composition
 
-Loading this package registers one effect-scoped Loader builtin. Each generated wrapper delegates to that builtin with its own module URL and prepared manifest. Repository skill roots mount as a uniquely named `dsh-skill-local` provider with default project/user roots excluded and watching disabled; cached package generations are immutable. Wrapper disposal removes the provider and all composed MCP clients through normal Cordis child-fiber teardown.
+Loading this package registers one effect-scoped Loader builtin. Each generated wrapper delegates to that builtin with its own module URL and prepared manifest. The runtime validates every declared skill root as an existing in-package directory before mounting — a package whose generated outputs were dropped (a `files`/`.npmignore` mistake, a damaged cache entry) fails the plugin load instead of silently mounting a skill-less plugin. Repository skill roots mount as a uniquely named `dsh-skill-local` provider with default project/user roots excluded and watching disabled; cached package generations are immutable. Wrapper disposal removes the provider and all composed MCP clients through normal Cordis child-fiber teardown.
 
 ## Common MCP format