Use ddev-assistant add-ons for AI coding assistants instead of installing them ourselves
## Summary
Instead of installing AI coding assistants (Claude Code, Codex, etc.) ourselves in the installer, use the maintained DDEV add-ons from e0ipso and let the user pick which assistants they want.
## Proposed add-ons
- `ddev-assistant-claude` — https://github.com/e0ipso/ddev-assistant-claude
- `ddev-assistant-codex` — https://github.com/e0ipso/ddev-assistant-codex
- `ddev-assistant-copilot` — https://github.com/e0ipso/ddev-assistant-copilot
- `ddev-assistant-cursor` — https://github.com/e0ipso/ddev-assistant-cursor
- `ddev-assistant-opencode` — https://github.com/e0ipso/ddev-assistant-opencode
## Behaviour
- Prompt the user (skipped under `-yolo`) for which assistants they want, then `ddev add-on get` **only the selected ones**.
- Delegating to these add-ons means we no longer maintain our own assistant install logic.
## Fewer restarts
Today the flow restarts DDEV more than it needs to — on Linux, `install-linux.sh` runs `ddev start` (line 235), then `ddev stop` + `ddev config` + `ddev start` (lines 251–253). Adding assistant add-ons *after* start would add yet another restart.
Add-ons should be fetched **before** the first `ddev start` so the project comes up already configured — targeting **one or zero** restarts total.
## Related
- #3601336 — Support declaring DDEV add-ons in the installer configuration file
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