Delete the oldest report files to stay under the size cap (closes #54)

When a report would take the report files past DATA_DIR_MAX_BYTES,
reportbuf now deletes the oldest report files until it fits, and does
the same at start when files left by an earlier run are already past
it. A file joins the files that may be deleted, at its place by name,
only once it is completely written, so a file still being written is
never deleted. A
report is refused with 507 only when the reports waiting to be written
fill the cap on their own, and then no file is deleted. The reports of
a failed write stop counting, and the part of its file written is
removed. A file whose deletion fails keeps counting; one already
deleted by hand counts as freed.

Model: opus-5-5

TODO.md

@@ -23,6 +23,15 @@ latest run passes.
 
 # Completed Steps
 
+- 2026-10-03: `DATA_DIR_MAX_BYTES` is now how much of the report files is kept
+  (issue #54): when a report would take them past it, the oldest report files
+  are deleted to make room, each deletion logged, and at start files already
+  past it are deleted the same way. A file still being written is never deleted.
+  A report is refused with 507 only when the reports waiting to be written fill
+  the cap on their own, and then no file is deleted. The reports of a failed
+  write stop counting, and the part of its file written is removed. A file that
+  cannot be deleted still counts until the next start; one already deleted by
+  hand counts as freed
 - 2026-10-03: `backend/script/lint` says what went wrong with its
   `.golangci.yml` check (issue #34). On a hash mismatch it says to compare the
   file with the org standard: if they differ, restore the org standard; if they