cli: sift batch submit|status|get|cancel; README and skill

Add `sift batch` so agents reach the batch endpoint through the CLI like
`fetch` and `search`. `batchcmd.go` has `parseBatchArgs` (verb plus the
unchanged args after it; submit takes one arg, status a job id optionally
preceded by `--results`, get a job id plus a non-negative-integer idx, cancel
a job id, else a usage error), `batchMain` dispatch, one small function per
verb, and `batchDo` (bearer header, decode the envelope, print it on stdout,
on non-2xx the error to stderr and `exitFor`). `submitMain` reads the file or
stdin and validates locally: a JSON object with a non-empty `targets` array,
or a bare array of targets or URL strings wrapped into `{"targets": [...]}`.

README gains a "Batch" section (endpoints, serial-per-job/one-worker/
round-robin model, caps, idempotency, lease/TTL/restart, summary-vs-results,
redaction, metrics); SKILL.md gains a "Batch" section.

Co-Authored-By: ornith-1.5-35b-a3b <ornith-1.5-35b-a3b@llama.cpp.invalid>

skill/SKILL.md

@@ -55,6 +55,33 @@ Useful flags: `--mode digest|clean|raw` (default `digest` when `--question` is g
 else `clean`), `--no-context` (skip Honcho lookup), `--recover` (server-side archive ladder
 for blocked pages), `--json` (full envelope: content, source, context, sizes, warnings).
 
+## Batch
+
+When you will read the content of several URLs later (a shortlist to compare), submit them once
+instead of a loop of `sift fetch`:
+
+```bash
+# one JSON object, or a bare array of URLs
+sift batch submit targets.json
+# ["https://a", "https://b"] also works; each string becomes {"url": ...}
+```
+
+Then poll and read:
+
+```bash
+sift batch status <job>          # summary: per-target status, no content
+sift batch status --results <job>  # everything at once — can be large, avoid unless needed
+sift batch get <job> <idx>       # one target's result, as `sift fetch --json`
+sift batch cancel <job>          # stop a queued job
+```
+
+- Submit once; a retry of the same list from this caller returns the same job (`reused`), it does
+  not start a second.
+- Don't use `--results` unless you need every result at once — a batch of 32 can be several
+  megabytes. Read each piece with `sift batch get <job> <idx>`.
+- A batch is slow on purpose: it runs one target at a time per job. For one URL you need now, use
+  `sift fetch`.
+
 ## PDFs
 
 PDFs work: `sift fetch` extracts them and returns the text. A short one comes back whole; a