session new: create a session at a chosen position (after/before another)

[olomix]

Right now session new always appends the new session to the end of the workspace. When I want a new session right after the current one, I create it and then repeatedly session move --to up until it lands in place.

With only a few sessions that's fine, but I usually have dozens. Then the walk-up takes many move calls, and it's visually noticeable in the sidebar as the session hops upward one step at a time. It would be much nicer to place it in one shot.

I see two possible shapes and would like your take on which fits agterm better:

Option A — reference-based flags on session new (and session move)
Add --after <sid> / --before <sid>:

agtermctl session new --after <current-sid>
agtermctl session move --after <sid> --target <sid>

Creates (or moves) the session directly at the target position — no follow-up moves.

Option B — extend the existing --to vocabulary
session move already takes --to up|down|top|bottom. Add reference targets to it, e.g. --to after:<sid> / --to before:<sid>, and let session new accept the same --to:

agtermctl session move --to after:<sid> --target <sid>
agtermctl session new --to after:<current-sid>

This stays inside the vocabulary that already exists — just new values for --to — and collapses the placement to a single move.

Either works for my use case — a session placed after the current one, without the visible step-by-step shuffle.

Questions:

1. Do you think this is worth adding?
2. If so, which variant do you prefer — new --after/--before flags, or extending --to with after:<sid>/before:<sid>?

Happy to send a PR once the direction is clear.

[umputun]

yes, worth adding. The append-then---to up walk is a real O(N) gap over the control channel, and the precise-placement primitive already exists internally (moveSession(_:toWorkspace:at:) with a clamped at index: - what drag-drop uses), so it's cheap to expose.

go with Option A - --after <sid> / --before <sid> on both session new and session move. Not Option B: --to is a closed keyword enum shared by session.go, session.move, and workspace.move, all pure enum decodes with no payload. Smuggling a sid into after:<sid> forces the parser to prefix-split-then-resolve and diverges session.move --to from the other two. Dedicated --after/--before keep --to clean and put the reference in its own value flag, same as --target/--workspace - which means it reuses the existing addressing sugar (id / unique prefix / active), so the nice case falls out for free:

agtermctl session new --after active

= create and place right after the current session, one round-trip.

for the PR:

• reuse moveSession(_:toWorkspace:at:) plus the drop-index resolver in SidebarDrop - the "after this row" index math and the post-removal off-by-one are already there and table-tested, no new arithmetic.
• no new command, just args on the two existing ones - keep it dispatcher-first (arg validation + error strings in ControlDispatcher, the resolve-and-move behind ControlActions).
• one thing to define: mutual exclusion on session move (--after/--before vs --to vs the positional workspace relocate), and what an anchor in a different workspace means. The primitive already supports cross-workspace placement, so --after <sid> could just drop the session right after that sid wherever it lives, or you keep v1 same-workspace-only - your call, just pin it down.

usual keep-in-sync (protocol round-trip + e2e + agent-skill docs). Send it when ready.

[olomix]

Shipped in #134 (merged). Thanks for the design guidance — went with Option A, --after/--before on both session new and session move, cross-workspace anchor supported. Closing as resolved.