[codex] Redact tool call log arguments

[Dreamstore2046]

Summary

Closes #242.

Replaces arbitrary 100-character tool argument truncation with explicit structured redaction:

• adds SAFE_TOOL_LOG_FIELDS with a per-tool allowlist
• redacts all non-allowlisted fields as <redacted: field_name>
• defaults unknown tools to redact every input field
• updates agent/server.ts to log structured redacted args instead of sliced JSON
• documents the convention in docs/observability/redaction.md
• adds Vitest coverage for every allowlisted tool shape, structured logger output, and unknown-tool defaults

Validation

• npm test -- agent/__tests__/tool-log-redaction.test.ts
• npx tsc --noEmit --target ES2022 --module ESNext --moduleResolution bundler --allowImportingTsExtensions --esModuleInterop --allowSyntheticDefaultImports --strict --skipLibCheck --types node,vitest agent/tool-log-redaction.ts agent/__tests__/tool-log-redaction.test.ts
• git diff --check

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