feat(mcp): add structured output schemas for existing tools

[JSONbored]

Parent roadmap: #127
Parent phase: #269
Parent epic: #273

Goal

Add structured output schemas for existing MCP tools so modern clients can validate and render Gittensory responses reliably.

Requirements

• Define schemas for existing machine-readable MCP tool outputs.
• Preserve existing CLI JSON behavior while adding MCP-native schema metadata.
• Keep schema fields public/private aware where outputs can cross surfaces.

Acceptance criteria

• MCP clients can discover schemas for the covered tools.
• Existing stdio smoke tests still pass.
• Schemas do not expose private fields through public-safe tools.
• This is not an autonomous PR bot and not public scoring.

Validation expected

• MCP schema discovery tests.
• npm run build:mcp.
• npm run test:mcp-pack.
• npm run test:ci.