feat(mcp): add structured usage and tag metadata tools

[Fail-Safe]

Summary

• Add structured MCP usage context via cortex_usage while keeping get_instructions concise for agent guidance.
• Add set_trace_tags and append_trace_tags for retrieval metadata hygiene.
• Add a tag_update metadata event path so long-tier tag changes avoid immutable trace-row fields.

Checks

• git diff --check
• go test ./...
• go build ./...
• go vet ./...
• go test -race ./...