v0.5.1 — fix MCP search budget

• Fixed: the MCP server's default token budget was a third of the CLI's, so
  search_document returned a single chunk per call. Chunks routinely reach
  ~1,000 tokens and the budget was 2,000, which left no room for a second one.
  In practice one question against a 34-page report cost six tool calls, each
  handing back one fragment — spending more tokens in total than the wider
  budget would have. The default is now 5,000, matching docsift search, and a
  test holds the two surfaces together.