feat: read split input from stdin (pipe support)

[NickMoignard]

What

yamlx split now reads its document from stdin when the argument is - or omitted while piping, so it composes with other tools:

# AWS CLI defaults to JSON — ask for YAML, then split the stream straight in.
aws quicksight describe-data-set \
  --aws-account-id "$ACCOUNT_ID" --region "$REGION" \
  --data-set-id "$DATASET_ID" --output yaml \
  | yamlx split - --out-dir dataset/

Why

Previously split required a file-path argument, forcing a temp file to bridge any pipeline. The engine already read through an io.Reader (yamltree.Load), and split output is named from identity keys rather than the input filename, so no real file is needed.

How

• internal/split/split.go — new RunReader(io.Reader, srcName, outDir, opts); Run(path, …) is now a thin file-opening wrapper over it. The os.Open failure stays in Run, so a missing file still surfaces as fs.ErrNotExist → usage error (exit 2).
• internal/cli/commands.go — split takes 0-or-1 args; reads stdin on a literal - or an auto-detected pipe (reusing the existing isTerminal helper + cmd.InOrStdin()). A bare interactive invocation is a usage error rather than a silent wait on stdin.
• Docs + tests: README piping example with the --output yaml caveat; RunReader unit tests and CLI tests for -, auto-detect, malformed-stdin (exit 1), and missing --out-dir (exit 2). Help golden regenerated.

Scope

split only. join consumes a directory tree (not a stream) and verify reads its input twice, so both are out of scope.

Verification

go build, go vet, golangci-lint (0 issues), and go test ./... all pass. Manually verified: explicit -, auto-detected pipe, malformed stdin → 1, missing --out-dir → 2, path still works, missing file → 2.

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