bug: opencode_ask_cmp missing '@' triggerCharacter, textEdit range, and returns '@' items on non-'@' words #343

Prerequisites

• I searched the docs
• I searched existing issues

:checkhealth opencode output

opencode: OK
- Neovim: 0.11+
- opencode CLI: available
- snacks.nvim: enabled (with blink.cmp)

Describe the bug

In lua/opencode/ui/ask/cmp.lua, the in-process LSP server (opencode_ask_cmp) for the ask() input buffer has three completion issues when paired with completion engines like blink.cmp or nvim-cmp:

1. Missing triggerCharacters = { "@" }:
   handlers[ms.initialize] only returns completionProvider = { resolveProvider = true }. Because @ is not a standard keyword character (iskeyword), typing @ in ask() does not automatically trigger the completion menu unless triggerCharacters = { "@" } is advertised.
2. Unconditional @ items on every word / path:
   handlers[ms.textDocument_completion] returns all @ placeholders (@this, @buffer, @diagnostics, etc.) unconditionally on every completion request, even when the user is typing normal prose or a file path (e.g. ./lua/...), causing @ context items to rank above path or buffer completions.
3. Missing textEdit replacement range covering @:
   Completion items set insertText = placeholder (e.g. "@buffer") without a textEdit range starting at the @ character. Because completion engines compute keyword bounds starting after @ (e.g. at b in @b), accepting @buffer after typing @b can produce @@buffer unless a textEdit range covering the leading @ is provided.

Steps To Reproduce

1. Enable blink.cmp (with lsp, path, buffer sources) for opencode_ask.
2. Open require("opencode").ask() and type @ — completion does not trigger automatically until a keyword character is typed or <Tab> is pressed.
3. Type ./ to complete a file path — @this, @buffer, @diagnostics appear above the filesystem paths because opencode_ask_cmp returns them unconditionally.

Expected Behavior

1. opencode_ask_cmp should advertise triggerCharacters = { "@" } in completionProvider.
2. textDocument/completion should only return @ context items when the token before the cursor starts with @ (or when manually invoked on an empty token), and include a textEdit range starting at the @ column.