[linyiru]

Each runner's serial console was copied to its own file under --log-dir:

• with no size limit;
• with no retention;
• with write errors swallowed on purpose, so that a failing log never stops the console copy.

A job writing to the console could therefore fill the host's disk, and the logs of finished runners piled up forever.

Changes:

• Per-runner cap: each file keeps at most --log-max-mib / FR_LOG_MAX_MIB (default 64), then one line saying the rest was dropped.
• Directory budget: before a new runner's log is opened, the oldest .log files are removed until all of them fit in --log-dir-max-mib / FR_LOG_DIR_MAX_MIB (default 2048). Other files in the directory are never touched.
• Permissions: the files are created 0600 and the directory 0700, since the console can carry what a job printed.
• Validation: both values must be ≥ 0, and the per-runner cap cannot exceed the directory budget.
• README: it now also says that the console reaches the daemon's own log at --log-level debug.

Tests are in internal/provisioner/consolelog_test.go and internal/config/config_test.go. scripts/mutate.sh confirmed that removing the cap, or removing the pruning, makes them fail.