fix: clear empty husks for pending prompts; if no graph, debug recall log, not error

[siillee]

claude-code + codex, identical changes.

• pending/ husks: pop_pending_prompt() now unlinks the buffer file when the
  last entry is consumed instead of writing {} back (one 2-byte husk per session,
  forever). A pop against nothing no longer creates a file. The SessionStart sweep
  also removes existing {} husks immediately (pending_husks in state_sweep).
• graph-scope 404: a 404 from /api/v1/recall on the ["graph"] scope — a fresh
  dataset with no graph yet — is logged as recall_graph_not_built and kept out of
  recall_error and the health accounting. Every other scope and status code is
  reported as before.

[github-actions]

Two file-cleanup fixes that somehow avoided deleting production — a modern miracle given the unlink count. 🗑️

🟢 No blocking issues found.

Minor observations:

• The graph 404 special-case assumes all 404s mean "dataset not built yet" rather than checking the error body for DatasetNotFoundError — could theoretically hide endpoint routing issues, though the error still logs to recall_graph_not_built so it's observable.

See inline comments for details.

No fixes needed — ship it.