Simplify CLI help with per-command topics

Replace the ~108-line monolithic --help with a concise 25-line overview
and add 18 per-command help topics (symbols, replace-symbol, insert-*,
replace-lines, insert-at, replace-content, references, rename, watch,
serve, mcp-serve, status, clean, init) plus concept topics (hashlines,
name-paths, languages, lsp). An LLM can now scan available tools
instantly and drill into any command with 'codescan help <command>'.

Author: pmarreck