fix(UX): improve prompt completions for VS Code slash commands

Addresses three UX issues with MCP prompt argument completions:

1. Database discovery: completeDatabasePath now scans $HOME/codeql/databases/
   as a default search location, and recursively discovers directories
   containing codeql-database.yml (including .testproj dirs) in the workspace.

2. Language auto-derivation: For prompts where queryPath is provided,
   language is now optional and auto-derived from the nearest
   codeql-pack.yml's codeql/<lang>-all or codeql/<lang>-queries dependency.
   Schema field ordering now puts queryPath before language so VS Code
   prompts for the query first. Affected schemas: explainCodeqlQuerySchema,
   documentCodeqlQuerySchema, workshopCreationWorkflowSchema,
   qlLspIterativeDevelopmentSchema.

3. Query path completion filtering: completeQueryPath now skips .github,
   dist, coverage, and build directories in addition to node_modules, .git,
   and .tmp — preventing workshop examples and build artifacts from
   cluttering the VS Code dropdown.

New tests: 19 unit tests covering all three fixes.
Updated tests: 9 existing tests updated to reflect language being optional.

Author: data-douser