Add concise discovery summary output

[five0nit]

Problem

brief2ship discover writes complete Markdown and JSON receipts, but terminal users often need one short summary immediately after a run.

Smallest useful outcome

Add an explicit output option that prints a concise, stable summary containing:

• overall disposition;
• selected/top candidate;
• score and evidence coverage;
• hard-blocker count;
• receipt paths.

The existing full receipt files remain authoritative.

Acceptance checks

• option name and help text are clear;
• deterministic fixture covers reuse, build-clean, and source-failure outcomes;
• concise output never hides a hard blocker or failed provider;
• stdout remains UTF-8 and machine-safe;
• existing default behavior remains backward compatible;
• documentation includes one example;
• full release validation passes.

Non-goal

Do not add a TUI, progress framework, or new runtime dependency.