fix(cli): tb export mentions agency buyers on an agency-only DB (#143)

[alexwolson]

Closes #143.

The bug

counts['solicitation'] in _cmd_export's success line counts the City spine alone. On a DB
that has agency data (agency_solicitation/agency_award/agency_bid + the export buyers
section) but no City-spine sync data, tb export printed Exported 0 solicitations to <path>
— even though it correctly wrote a full buyers section. A successful export read as though it
did nothing.

The fix

Matches the issue's own suggested wording: mention the agency buyer count alongside
solicitations when it's non-zero.

buyers = counts.get("buyer", 0)
detail = (f"{counts['solicitation']} solicitations + {buyers} agency buyer(s)"
         if buyers else f"{counts['solicitation']} solicitations")
print(f"Exported {detail} to {written}")

buyer is only ever non-zero once agency enrichment has seeded it (seed_buyers), so the plain
City-spine case — the overwhelming majority of real exports — is completely unaffected: no
+0 agency buyer(s) clutter on a run that's never touched agencies.

Verification

Live, both cases:

agency-only DB:  Exported 0 solicitations + 3 agency buyer(s) to .../bids.json
plain fresh DB:  Exported 0 solicitations to .../bids.json

2 new tests, confirmed genuinely red against the pre-fix code by reverting the fix in place:
the agency-only assertion fails with exactly the string this issue reports, while the plain-DB
test stays green throughout — proving the fix is targeted. Restored after.

776 tests passing.

🤖 Generated with Claude Code

https://claude.ai/code/session_01W53WHx8mm2UHuLFAQWeF62