server: v2 removed /tui/append-prompt — external clients can no longer pre-fill the TUI input #54681

Summary

OpenCode v1 exposed TUI control endpoints, notably POST /tui/append-prompt
("Append prompt to the TUI", used by editor integrations to insert context
references like path/to/file.lua:L10-L20 into the prompt input so the user can
edit before sending). v2.0.26 removed all /tui/* routes without a replacement,
so external clients have no supported way to pre-fill the TUI input box without
immediately sending a prompt.

Environment

• opencode version: 2.0.26
• OS: Linux 6.6.87.2-microsoft-standard-WSL2 (x64)
• Terminal: xterm-256color, panes managed by zellij 0.44.3
• Shell: zsh
• Install/channel: release (opencode.ai installer)
• Active plugins: none relevant (the affected integration is the third-party
  editor plugin nickjvandyke/opencode.nvim connecting via service.json)

Reproduction

1. Start opencode v2 (TUI + background service).
2. Read the service URL/password from ~/.local/state/opencode/service.json.
3. Call the v1 route: curl -u opencode:$PASSWORD -X POST "$URL/tui/append-prompt" -d '{"text":"hello"}'
4. Check the running server's own spec: curl -u opencode:$PASSWORD "$URL/openapi.json" | grep -i tui

Expected Behavior

A supported v2 way for external clients to append text to the connected TUI's
input without sending — e.g. POST /api/tui/append-prompt (or a session-scoped
equivalent) that forwards the text to the foreground TUI client over the event
stream, restoring v1's client.tui.appendPrompt capability.

Actual Behavior

• The running server's GET /openapi.json lists 116 routes; none are TUI
  control routes.
• POST /tui/append-prompt returns 405 — but so does POST to any
  non-existent path (the SPA catch-all), and GET /tui/append-prompt returns
  the web UI HTML, i.e. the route does not exist.
• POST /api/session/{sessionID}/prompt always sends immediately; the prompt
  input is client-local state with no server-side API to touch it in v2.

Additional Context

• v1 reference: client.tui.appendPrompt → POST /tui/append-prompt
  (sst/opencode-sdk-js, -python, -go, TuiAppendPromptParams).
• Current workaround in opencode.nvim (verified working but fragile): locate
  the TUI process via /proc (cmdline opencode + cwd match), read
  ZELLIJ_SESSION_NAME/ZELLIJ_PANE_ID from its environ, then inject
  keystrokes with zellij action write-chars -p <pane> "<ref>"; or
  chansend() when the TUI runs inside an nvim embedded terminal. This is
  multiplexer-specific and impossible for TUIs in plain external terminals.
• Issue is deterministic, not intermittent.