feat: export --quiet suppression and --json no-op (TDD)

[asphaltbuffet]

Parent

#170

What to build

Add two flags to the export command:

• --quiet / -q: suppresses the zero-events warning on stderr. The stdout data stream is always unaffected.
• --json: accepted silently as a no-op. The command always emits NDJSON; this flag exists so users who pass it habitually don't get an unexpected error.

Write tests first (red → green), extending the table in cmd/export/export_test.go:

Case	Verify
Zero events + -q	stdout empty, stderr silent, exit 0
--json flag present	command does not error; output is still NDJSON

Acceptance criteria

• Tests written before implementation (TDD)
• --quiet / -q flag wired; zero-events warning suppressed when set
• --json flag accepted without error (no-op)
• stdout data stream unaffected by --quiet
• All new tests pass
• mise run lint and mise run test clean

Blocked by

• feat: export NDJSON stdout output + zero-events warning (TDD) #174