Demote per-file directive skip logs to DEBUG (M13.5)

[espadonne]

Summary

Audit 13 M13.5: `dlm show` on a 2,021-file source-directive document
emits 243 `directive: ... exceeds max_bytes_per_file=...; skipping`
lines to stderr — once per oversize file, every invocation. The
audit explicitly flagged this as a 1-2s/call hit + log noise.

This change demotes all per-file skip logs in `directives/expand.py`
from INFO to DEBUG:

• size-cap (`exceeds max_bytes_per_file`)
• max_files truncation
• binary file (NUL in first KiB)
• image / audio without blob_store
• audio without transcript

WARNING-level logs for genuine read/stat/UTF-8 failures stay loud —
those indicate something the user might want to fix. The summary
count is still surfaced via `provenance.skipped_*` (and via the
trainer's `directives: expanded N file(s) across M source(s)` line).

A separate caching layer for the expansion result is the right
follow-up to address the re-walk cost — out of scope for this
fix; tracked for a future sprint.

Test plan

• New caplog test asserts no INFO records on size-cap skip path
• All 16 existing expand tests still pass
• ruff + mypy clean