feat(skills): add nix style guidance

.pi/prompts/review-nix.md

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+---
+description: Review Nix changes against repository Nix style rules
+argument-hint: "[target]"
+---
+
+Use the `nix-style` skill.
+Audit ${ARGUMENTS:-the staged diff} for violations of the repository Nix style rules.
+
+Report only actionable findings.
+Order findings by severity.
+Do not report preferences unless a loaded rule states them.
+
+Use this format for each finding:
+
+```text
+`path:line`: Finding title.
+Summary: why the code breaks the rule
+Fix: suggested fix
+```
+
+It is acceptable to report no findings when the reviewed Nix already follows the loaded rules.

.pi/skills/nix-style

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+../../src/skills/nix-style