Expand context-engine SKILL documentation

Comprehensive rewrite and expansion of the context-engine SKILL docs: reorganized into Quickstart, Decision Tree, and detailed tool families (search, repo_search, symbol_graph, graph_query, pattern_search, batch tools, memory, admin). Adds full parameter references, examples, routing/performance guidance, memory workflow, error handling, best practices, cross-repo and admin diagnostics. Also updates related reference files and the skills/context-engine copy to align with the new guidance.

Author: m1rl0k