feat: add Allium pi.dev skills and upstream repository

Install Allium skills (allium, elicit, distill, propagate, tend, weed)
from the pi-allium-port, with corrected symlinks for upstream content.

Add juxt/allium as a git submodule for language reference materials.
Allium CLI v3.0.5 is already installed via Homebrew.

.pi/skills/elicit/references/language-reference.md

@@ -0,0 +1 @@
+../../../../allium-main/skills/allium/references/language-reference.md

.pi/skills/elicit/references/library-spec-signals.md

@@ -0,0 +1 @@
+../../../../allium-main/skills/elicit/references/library-spec-signals.md