init: a failing bd is reported as 'not on your PATH' #1361

Found by the weekly verify on 2026-11-01, on main@0460513 (reverify-init-images).

What happens

When bd is on PATH but bd version fails (non-zero exit, crash), init reports that "bd isn't on your PATH". detectHostBdVersion swallows the error and treats every failure as missing (src/cli.ts:199-208).

Repro

Put a bd on PATH that exits 1, then run sandcastle init --issue-tracker beads.

Acceptance

• "Not on PATH" is reported only when the binary is missing. A failing bd is reported as failing, with its stderr.
• A test covers both cases.

This issue was AI-generated (Claude Code) on behalf of @mattpocock.