feat(lanes): offer a create/update readme run when adding a lane

An optional checkbox in the Add-lane modal starts the new lane's terminal
run right after creation with `/sg-all:sg-md-writing` as its first message,
so the lane opens already writing or refreshing its CLAUDE.md/AGENTS.md set.
Claude lanes only — codex has no slash commands.

docs/LANES.md

@@ -43,6 +43,8 @@ The modal has a segmented toggle mirroring the CLI's two modes:
 - **Repo** — adopts the given directory as-is (maps to `agenthub lanes add --cwd`). No branch fields; the auto-setup summary above does not run (adopting is instant, nothing to wait on).
 - **Worktree** (default) — provisions a managed worktree (maps to `agenthub lanes add --repo`). Unlike the CLI, the modal requires you to type the new branch's name yourself rather than deriving one from the title — the underlying route still derives one when `branch` is omitted, so the CLI's behavior is unchanged.
 
+Under the *Pipeline template* select, a **Create/update readme file** checkbox (Claude lanes only — Codex has no slash commands) starts the new lane's terminal run right after creation with `/sg-all:sg-md-writing` as its first message, so the lane opens already writing/refreshing its `CLAUDE.md`/`AGENTS.md` set. It requires the `sg-all` plugin to be installed in Claude Code; without it the session simply reports an unknown command. Leave it unticked to create the lane without starting a run.
+
 Both modes' path field has a **Browse** button next to it, opening a small folder browser (`GET /api/lanes/browse?path=<abs>`) instead of a native OS picker — a browser cannot hand a web page an absolute filesystem path from a native dialog, so this dashboard (local-first, server and browser on the same machine) lists directories server-side instead: click a subfolder to descend, ".." to go up, "Select this folder" to fill the path field. Git repos are marked in the listing.
 
 ## Lane tool: Claude or Codex