fix(subagent): keep fleet-dry parent mail to a digest plus blob - #1237

Summary

• Fleet-dry continuation runs newly collected terminals through the same Summary + Blockers digest and report_uri as mailbox mail, instead of JSON.stringifying full reports.
• Unstructured reports keep a short inline teaser (UNSTRUCTURED_DIGEST_CHARS); the read_file pointer comes from the truncation notice already on main.
• Already-collected IDs stay id, status, and description only, and in-flight mailbox IDs are still omitted from the same parent window.

Verification

• Rebased onto main after fix(subagent): name mailbox digest blobs for parent fetch #1240 and fix(subagent): count mailbox spill remaining as omitted body #1243; bun run check passes (7880 tests, 0 fail).

Fixes CL-9494