refactor: consolidate five duplicate byte formatters into hermes_cli.sizefmt

Five modules each carried a private near-identical human-readable byte
formatter (backup._format_size, checkpoints._fmt_bytes,
doctor._human_bytes, context_references._human_bytes,
curator_backup.format_size). Three of them silently topped out at GB and
rendered a 1 TiB value as '1024.0 GB'. All five now alias one shared
format_bytes in hermes_cli/sizefmt.py (sibling of timefmt.py, same
zero-dependency rationale), keeping each module's established local name
so no caller churns.

Deliberately NOT migrated (behavior differs on purpose):
- session_recovery._format_bytes: binary suffixes (KiB/MiB/GiB)
- qqbot chunked_upload.format_size: '100.0 B' one-decimal style, pinned
  by its protocol tests

Net -33 production LOC before the new module; parity verified over a
16-value corpus against all five verbatim originals (only divergence:
the TB tier fix). Contract tests mutation-checked red-green.

agent/context_references.py

@@ -13,6 +13,7 @@ from typing import Awaitable, Callable
 
 from agent.model_metadata import estimate_tokens_rough
 from hermes_cli._subprocess_compat import IS_WINDOWS, windows_hide_flags
+from hermes_cli.sizefmt import format_bytes as _human_bytes
 
 _QUOTED_REFERENCE_VALUE = r'(?:`[^`\n]+`|"[^"\n]+"|\'[^\'\n]+\')'
 REFERENCE_PATTERN = re.compile(
@@ -554,15 +555,6 @@ def _rg_files(path: Path, cwd: Path, limit: int) -> list[Path] | None:
     return files[:limit]
 
 
-def _human_bytes(n: int) -> str:
-    size = float(n)
-    for unit in ("B", "KB", "MB", "GB"):
-        if size < 1024 or unit == "GB":
-            return f"{int(size)} {unit}" if unit == "B" else f"{size:.1f} {unit}"
-        size /= 1024
-    return f"{size:.1f} GB"
-
-
 def _binary_reference_block(ref: ContextReference, path: Path) -> str:
     mime, _ = mimetypes.guess_type(path.name)
     mime = mime or "application/octet-stream"