Protect skill projections from foreign collisions

[tomvaillant]

Summary

• preflight every project skill projection before writing any editor target
• preserve exact existing links idempotently
• refuse foreign files, directories, dangling links, and links to another source without modifying them

This prevents OpenKnowledge from silently deleting a user- or installer-owned skill when a same-name projection is installed. Preflighting all requested targets also prevents a partial multi-editor projection when one target conflicts.

Validation

• pnpm --filter @inkeep/open-knowledge-server typecheck
• pnpm --filter @inkeep/open-knowledge-server exec vitest run skill-projection.test.ts --pool=forks --maxWorkers=1 --no-file-parallelism --reporter=verbose
• pnpm exec biome check packages/server/src/skill-projection.ts packages/server/src/skill-projection.test.ts .changeset/safe-skill-projections.md --error-on-warnings

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