Mark read-only Docker Hub tools with readOnlyHint

[RomanOstr]

Summary

• add readOnlyHint: true to read-only Docker Hub MCP tools
• update generated tool metadata in tools.json

Why

Several Docker Hub tools are read-only, but they are currently published without readOnlyHint. MCP clients can then classify them as potentially mutating and prompt more aggressively than needed.

This patch marks the non-mutating account, repository, tag, hardened-image, and search tools as read-only.

Scope

This PR intentionally excludes fork-specific repo maintenance changes such as CI hardening, CODEOWNERS, release workflow changes, and Docker Hub publishing setup. It keeps the upstream proposal limited to the MCP metadata fix itself.

Validation

• npm run lint passed locally on Windows
• The modified files were reviewed directly against the current upstream sources
• Full upstream validation is expected to come from GitHub Actions on this PR

Note: local npm run build and npm run list-tools:check hit a Windows-specific Node heap exhaustion in this environment, so I am not claiming those as local passes.