[idea] herdr pane split --label <text> (atomic create-with-name like tab/workspace)

[hc-12]

Summary

tab create --label X and workspace create --label X both support atomic creation-with-name. pane split does not. Today, naming a freshly split pane requires a second call:

NEW=$(herdr pane split <id> --direction right --no-focus | jq -r '.result.pane.pane_id')
herdr pane rename "$NEW" "<label>"
herdr pane run "$NEW" "<cmd>"

vs the symmetric one-step form already available for tabs:

herdr tab create --workspace 1 --label "logs"

Proposal

Add --label <text> to herdr pane split:

herdr pane split [<pane_id>|--pane ID|--current] --direction right|down [--ratio FLOAT] [--cwd PATH] [--label TEXT] [--focus|--no-focus]

Behaviour identical to split followed immediately by rename — just removes the round-trip.

Motivation

Scripts that bootstrap multiple panes (test runners, dev servers, log tails, sibling worker panes) currently leave a window where the new pane appears unlabeled in the sidebar between the split response and the rename call. When several panes are spawned in a loop, several (unnamed) entries can stack up briefly until each rename lands. --label at creation closes that window and matches the existing tab/workspace API surface.

Related: #476 (Agents panel should follow herdr pane rename) — the underlying rename plumbing already exists; this is purely a CLI ergonomics + atomicity ask.

Workaround today

Two-step split → rename works fine, just not atomic and not symmetric with tab/workspace.

[jefflitt1]

Second data point for this, from the metadata side rather than the label side.

I hit the same window from automation that splits a pane and then stamps it with
pane report-metadata tokens so a later sweep can tell orchestrator-created panes from ones the
human opened. Between the split response and the stamp landing, the pane exists carrying
nothing, and if the caller dies in that gap it stays that way — which quietly breaks any rule of
the form "an unmarked pane belongs to the human."

So the same atomicity argument applies to --token k=v as to --label: one call that creates the
pane already marked. Same asymmetry too, since report-metadata is the only way to set tokens and
it is always a second round trip.

Still open on 0.8.2 — pane split takes --cwd, --env, --ratio, --direction,
--right-click, and focus flags, but no --label and no --token.

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