Two improvements to `agnes pull` progress reporting:
1. **Aggregated per-file progress across chunked downloads**: the
existing Rich progress bar already used one task per file, but the
chunked-download contract (one file = N parallel chunk callbacks
summing to file size) meant we needed to verify that all chunk
threads advance the same task. They do — the per-file callback is
constructed once per tid and routes every chunk's byte delta to the
same task / textual entry, so the bar shows one aggregated bytes-
downloaded total rather than N separate sub-bars.
2. **Textual fallback for non-TTY stderr**: when stderr is not a
terminal (SessionStart hook, CI runner, Docker log capture), Rich
either suppresses output (silent multi-minute pull on a 5 GB
parquet) or emits raw control sequences. The new `_TextualProgress`
helper instead emits one plain-text line per file at most every
10%-of-total-bytes or 30 s, plus a final `100% done` line per file.
Format: `[N/T files] <tid>: 25% (16 MB / 66 MB) at 1.5 MB/s`.
The TTY path is unchanged. Detection uses `sys.stderr.isatty()` —
`show_progress=True` flips into the textual fallback when that returns
False. `show_progress=False` (the SessionStart hook) still emits no
progress text in either mode.
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