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

examples/headless-agent/tests/fixtures/repository-plugin/dsh-plugin.mjs

@@ -1,9 +1,9 @@
 // Generated by dsh-plugin-prepare. Do not edit.
-const manifest = { "name": "headless-repository-fixture", "skills": ["dsh-plugin-assets/skills/0"] }
-export const name = 'headless-repository-fixture'
-export const inject = ['loader']
+const manifest = {"name":"headless-repository-fixture","skills":["dsh-plugin-assets/skills/0"]}
+export const name = "headless-repository-fixture"
+export const inject = ["loader","skills"]
 export async function apply(ctx) {
-  const runtime = ctx.loader.builtins['dsh-repository-plugin']
-  if (runtime === undefined) throw new Error('missing Cordis builtin dsh-repository-plugin')
+  const runtime = ctx.loader.builtins["dsh-repository-plugin"]
+  if (runtime === undefined) throw new Error("missing Cordis builtin dsh-repository-plugin")
   await ctx.plugin(runtime, { baseUrl: import.meta.url, manifest })
 }