feat(#69): nest concrete skills under namespace routers at install (#883)

Emit the 6 gsd-ns-* routers as the only top-level skill bundles and nest
the ~61 concrete skills under <router>/skills/<name>/SKILL.md on runtimes
with confirmed non-recursive skill loaders (claude global, cline, qwen,
hermes, augment, trae, antigravity). Router bodies rewrite their routing
tables from Skill-tool dispatch to a Read skills/<name>/SKILL.md pattern.
Recursive/unconfirmed loaders (cursor, codex, copilot, windsurf, codebuddy,
opencode, kilo) keep the flat layout. Completes the v1.40 namespace
architecture (#2792) so the eager skill listing drops to ~6 entries.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

scripts/lint-test-file-count.allowlist.json

@@ -123,6 +123,7 @@
         "bug-410-install-defaults-test-mode-guard.test.cjs",
         "enh-776-install-gemini-hook-events.test.cjs",
         "install-minimal-hooks.test.cjs",
+        "install-nested-layout.test.cjs",
         "install-path-detection.test.cjs",
         "install-regressions.test.cjs",
         "install-runtime-artifacts.test.cjs",