mq-mcp v1.6.0 — Generated artifacts + repo-signal merge

What's new

• Activated repo-signal pack merging in the callgraph builder — reads from .repo-signal/exports/, handles callgraph.v1, symbol_index.v1, repo_summary.v1, and risk_map.v1. Hub files refreshed from merged importers.
• Added generated/ directory structure: symbols/, callgraphs/, architecture/ with .gitkeep files; JSON snapshots excluded from version control.
• Added export_symbol_index MCP tool (Class C) — writes in-memory callgraph symbol map to generated/symbols/symbol_index.json.
• Added repo_signal_status MCP tool (Class A) — reports presence, schema, and age of each .repo-signal/exports/ pack.
• Updated docs/tool_contracts.json — now covers all 73 tools.
• Tool count: 71 → 73.