fix: validate MCP Toolbox and add Codex marketplace

[jpantsjoha]

What changed

• Added the Codex marketplace manifest with the GoogleCloud Plugin interface metadata.
• Replaced the obsolete uvx toolbox-core configuration with the supported @toolbox-sdk/server --stdio --config command in Claude, Gemini, and shared MCP configuration.
• Documented and directly validated the gcloud MCP and Toolbox MCP surfaces.
• Refreshed README positioning and persona count.

Why

toolbox-core is a Python library rather than an MCP server CLI. The previous configuration could not start Toolbox. The corrected package and arguments complete an MCP stdio initialization and expose the expected tools.

Impact

Codex can discover the plugin through the local marketplace, and configured agents can launch the Toolbox database MCP server with a supplied tools.yaml.

Validation

• make gate — passed: 17 skill contracts, packaging for Claude/Antigravity/Codex/Kimi, Mermaid, 100 links, and 136 smoke tests.
• JSON parsing and whitespace checks — passed.
• gcloud MCP v0.5.3: stdio initialize, tool discovery, and read-only project lookup — passed.
• Toolbox v1.7.0: stdio initialize and tool discovery with the BigQuery prebuilt configuration — passed.
• Security audit — no credential values committed; configuration references environment variables only.

Freshness note

make check completed with 9 fresh and 96 drifted source hashes, with zero fetch errors. This is a pre-existing repository-wide documentation freshness backlog outside this configuration-focused change; the affected Toolbox source was manually reviewed against the current official CLI documentation.