Add packages/mosaic registry schemas, validation, read-only CLI; pin pi 0.85.1 (#1499)

packages/mosaic/tests/fixtures/valid/auth/providers/ollama-remote.json

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+{"providerVersion":"1","id":"ollama-remote","kind":"custom-endpoint","allowInsecureTransport":true,"harnesses":{"pi":{"api":"openai-completions","baseUrl":"http://ollama.internal:11434/v1","models":["qwen2.5-coder:7b"]}},"credentialTypes":["none"]}

packages/mosaic/tests/fixtures/valid/auth/providers/openai-codex.json

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+{"providerVersion":"1","id":"openai-codex","kind":"native","harnesses":{"pi":{"providerId":"openai-codex"}},"credentialTypes":["oauth","api_key"]}