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

[TheGreatAxios]

Summary

• Fleet-dry continuation delivers the same Summary + Blockers digest and report_uri as mailbox mail, instead of JSON.stringifying full reports for newly collected terminals.
• Unstructured reports keep a short inline teaser; occupancy mail names read_file on each report_uri for the full blob.
• Already-collected IDs stay id, status, and description only, and in-flight mailbox IDs are still omitted from the same parent window.

Verification

• bun run typecheck, bun run build, and bun run test pass
• bun run check passes (7782 tests, 0 fail)

Fixes CL-9494

[linear-code]

CL-9494