feat(lsp): add completion method to LSPManager

[amondnet]

Summary

Add code completion support to LSPManager with full LSP 3.17 specification compliance. This enables AI-assisted code editors to provide real-time completion suggestions to users.

Changes

New Methods

• completion() - Get code completion suggestions at a given position in the file
  • Takes file path, line, and character position
  • Returns normalized CompletionItem[] array
  • Handles multiple language server clients

New Types

• CompletionItemKind enum - Standard completion item type classifications (Method, Function, Class, etc.)
• CompletionItem - Individual completion suggestion with label, kind, detail, and documentation
• CompletionList - Collection of completion items with incomplete flag

Implementation Details

• Sends textDocument/completion LSP request to language servers
• normalizeCompletions() - Normalizes responses to handle both:
  • CompletionList format with isIncomplete flag
  • Direct CompletionItem[] format
• isCompletionList() - Type guard for runtime validation
• Follows LSP 3.17 specification: https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#textDocument_completion

Test Plan

• Verify completion method is exported from @pleaseai/code-lsp
• Test completion with TypeScript language server
• Verify both CompletionList and CompletionItem[] responses are handled
• Test error handling when no clients available
• Verify type guards work correctly at runtime

Related Issues

Closes #15