`contextix export --format mermaid`

[kiyeonjeon21]

Emit a Mermaid flowchart for a subgraph.

$ contextix export --format mermaid --entity "Federal Reserve" --hops 2
graph LR
  A[Federal Reserve] -->|influences| B[Fed rate]
  ...

Acceptance:

• --format accepts json (default), mermaid, eventually cypher and d2
• --entity <name> + --hops <N> scopes the export
• Write to stdout; user can pipe to a file

Good first issue — one new file (src/export/mermaid.ts) plus CLI wiring.