Epic: Make co-owner (T3) sign-off auditable and safe #22

Goal

--allow-t3 is now global and opt-in (PR #20), but it is still a bare flag: nothing records or checks that sign-off exists, and the state it sets is process-wide. Make consent verifiable and scope-safe.

Exit criteria

• Sign-off is read from a recorded, reviewable source.
• The flag cannot leak across invocations or threads.
• Docs describe the real command surface.

Scope

• E1.1: T3 sign-off is trust-only: no record is read or verified
• E1.2: T3 approval is process-wide state (not safe for concurrent or embedded use)
• E1.3: Land PR Add opt-in T3 showcase support and Pick Your AI identity #20 and document the global --allow-t3

Progress is tracked through this issue's sub-issues.

Prior work

Prior work: open PR #20 (opt-in --allow-t3 and the Pick Your AI family), open #7 and PR #13 (variant J badges), merged PRs #14 to #19 (icon art-direction path, UTF-8 output, registry-free org health and label planning, Windows test portability, footer URL repair) and closed issues #8 to #12. Nothing here duplicates those; where an item is already handled it is linked, not refiled.

Fresh VM verification: not run. Findings were verified on a Windows host; hosted CI is separate evidence.