v0.5.1

Bug fixes from real-world agent pipeline testing.

• Fixed --formula with --json and --from-csv — no longer prepends = to all strings in batch mode. Plain text stays plain; only strings starting with = are written as formulas. Single-cell --formula behavior unchanged
• Fixed --columns by header name when combined with --range — row-1 headers are now resolved even in range-scoped searches (both Polars and openpyxl paths)
• Fixed --headers on multi-range reads — column letters are now resolved to row-1 header names with per-sheet caching, matching single-range behavior
• Fixed resolve_column_letters() to prioritise header name lookup over column letter matching — prevents purely alphabetic header names (e.g. "Formula") from being misinterpreted as column letters
• Updated --formula description in command reference to reflect batch vs single-cell semantics
• Added 6 regression tests for mixed formula/text writes, header name + range search, and multi-range header resolution