feat(gadgets): add multi-mode EditFile and line numbers to ReadFile

[zbigniewsobiecki]

Summary

• EditFile multi-mode: Refactored to support 3 operation modes via discriminated union:

  • search_replace: Existing behavior with layered matching (exact → whitespace → indentation → fuzzy)
  • insert_at_line: Insert content at a specific line number (1-based, inserts BEFORE the specified line)
  • remove_lines: Remove a range of lines (1-based, inclusive)

• ReadFile line numbers: Added optional showLineNumbers parameter that prefixes each line with its 1-based line number (e.g., 1 | content) for precise line-based editing

Changes

EditFile (src/gadgets/EditFile.ts)

• Schema changed from flat object to z.discriminatedUnion('mode', [...])
• Updated description documenting all three modes
• Refactored execute() to dispatch to mode-specific handlers
• Added 6 new examples (3 for insert_at_line, 3 for remove_lines)
• All existing examples updated with mode: 'search_replace'

ReadFile (src/gadgets/ReadFile.ts)

• Added showLineNumbers optional boolean parameter (default: false)
• Added addLineNumbers() helper method
• Added example showing line numbers output

Edge Cases Handled

Scenario	Mode	Handling
Empty file	insert_at_line	Insert at line 1
Empty file	remove_lines	Error: startLine beyond EOF
Line > EOF	insert_at_line	Append at end
startLine > endLine	remove_lines	Error
startLine > EOF	remove_lines	Error

Test plan

• All existing tests pass (120 tests)
• TypeScript type checking passes
• Biome lint passes
• Pre-commit and pre-push hooks pass

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