refactor(BUZZ-231): clarify audience helpers and share json output

[sarahweeks-prolific]

Summary

Follow-up to #527 for Matt's post-merge comments on audience count / audience breakdown.

Call sites no longer pass a naked bool into shared helpers. --json is registered once with the other audience flags. Both commands emit the client/API response type for --json instead of count inventing its own DTO.

Implementation

• Split flag setup into addCountFlags / addBreakdownFlags and validation into validateCount / validateBreakdown. The bool stays inside filter_input.go.
• Move -j/--json into the shared flag registrar (still hand-bound — shared.AddOutputFlags would steal -t from --template-path).
• Count --json now marshals client.EligibilityCountResponse, matching breakdown's use of client.FilterBreakdownResponse. CountResult is gone. Wire JSON is still {"count":N}.

Testing

Existing cmd/audience tests cover the same paths; no behaviour change. go test ./cmd/audience/... passes.

[prolific-snyk]

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[Copilot]

Copilot review overview

🟢 Approval recommended

The focused refactor preserves existing behavior and has no unresolved review findings.

Review effort: Balanced
Findings: None

What changed in this PR

Refines the audience commands introduced in #527 by clarifying shared helpers and consolidating JSON output handling.

Changes:

• Adds command-specific flag and validation helpers.
• Centralizes --json registration and uses the API response type for count output, preserving {"count":N}.

File	Description
cmd/​audience/​filter_input.go	Adds named helpers and shared JSON flag binding.
cmd/​audience/​count.go	Uses count helpers and the API response type.
cmd/​audience/​breakdown.go	Uses breakdown helpers and shared JSON state.

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