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/allium/references/actioning-findings.md

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+../../../../allium-main/skills/allium/references/actioning-findings.md

.pi/skills/allium/references/assessing-specs.md

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+../../../../allium-main/skills/allium/references/assessing-specs.md

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

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+../../../../allium-main/skills/allium/references/language-reference.md

.pi/skills/allium/references/migration-v1-to-v2.md

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+../../../../allium-main/skills/allium/references/migration-v1-to-v2.md

.pi/skills/allium/references/migration-v2-to-v3.md

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+../../../../allium-main/skills/allium/references/migration-v2-to-v3.md

.pi/skills/allium/references/patterns.md

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+../../../../allium-main/skills/allium/references/patterns.md

.pi/skills/allium/references/test-generation.md

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+../../../../allium-main/skills/allium/references/test-generation.md