feat: batch file ingestion (weave docs create-batch)

Problem

There is no native multi-file ingestion command. Client0 wrote a 255-line bash script to
process 20 PDFs sequentially with retry, Milvus restart, and skip/resume logic. This
complexity belongs inside weave-cli.

Proposed Solution

New subcommand `weave docs create-batch`:

```bash
weave docs create-batch AuctionListings "data/pdfs/*-catalogue.pdf"
--milvus-local
--embedding text-embedding-3-small
--skip-existing
--timeout 30m
--delay 10s
--max-retries 3
--log-file logs/ingestion.log
--checkpoint-file .ingestion-checkpoint.json
```

Key Features

1. Glob expansion — already in v0.9.27
2. Configurable delay between files (`--delay 10s`) — prevents VDB memory buildup
3. Retry with backoff (`--max-retries 3`, `--retry-delay 30s`) — auto-retry failed files
4. Checkpoint/resume (`--checkpoint-file`) — save state after each file, resume on crash
5. Structured log file (`--log-file`) — timestamped append-mode log
6. Auto-create image collection — if `--image-collection` doesn't exist, create it
7. Batch summary at completion:
   ```
   ✅ Batch complete: 9/9 succeeded, 0 failed, 2 skipped
   Duration: 1h 23m | Log: logs/ingestion.log
   ```

Checkpoint File Format

```json
{
"collection": "AuctionListings",
"started": "2026-02-17T10:00:00Z",
"completed": [
{"file": "2017-catalogue.pdf", "chunks": 28, "at": "2026-02-17T10:12:33Z"}
],
"failed": [],
"skipped": []
}
```

Client0 Impact

Replaces the entire 255-line bash wrapper. Two `weave docs create-batch` calls replace the
entire pipeline script.

Priority

P1 — v0.9.29 target (~10 hours)

Start by locating the existing weave docs create subcommand and the v0.9.27 glob-expansion path. Read how one file is ingested, how Milvus and image collections are handled, and where CLI options are defined. Done means the new command supports the listed delay, retry, checkpoint, logging, skip/resume, collection, and summary behaviors with tests for the batch flow.