feat: route compact reads through unified query

[jlitola]

Summary

• add a transport-neutral ReadService that sends one backend Query.read
  request and validates the exact code/docs union contract
• route advertised MCP read and compact top-level CLI reads through the
  unified operation while retaining deprecated commands and --repo-url on
  legacy roots for observable compatibility usage
• require McpToolServices.readService, export the client through
  @githits/mcp/client, and document the custom-endpoint and hosted-MCP rollout
  boundaries

Verification

• bun test — 4,742 passed, 0 failed
• bun run typecheck
• bun run format:check
• bun run lint — passes with existing repository-target warnings
• bun run build
• bun run validate:packages
• bun run validate:packages:mcp-publish
• bun run smoke:cli:built
• bun run smoke:mcp:built
• authenticated built CLI compact code/docs probes against production
• authenticated built local MCP compact code/docs probes against production
• internal code review clean
• Claude Opus external review clean in round 2

Broad authenticated CLI/MCP smokes passed their relevant registration/read
checks but did not complete because the unrelated REST language endpoint hung or
timed out. The changed GraphQL path was isolated and passed all four production
probes above.

Rollout boundary

The hosted MCP still requires a separate @githits/mcp release, remote-mcp
dependency update/provider wiring, and deployment. This PR does not merge,
release, publish, or deploy that host.