fix: improve delta spec validation with case-insensitive headers and empty section detection

[TabishB]

Summary

• Delta section headers are now parsed case-insensitively (e.g., "Added Requirements", "ADDED Requirements", and "added requirements" all work)
• Empty delta sections now produce clear error messages guiding users to add requirement entries
• Specs with no delta headers at all now receive specific error messages explaining that delta sections are required
• Added test coverage for case-insensitive delta header parsing

Implementation Details

Modified src/core/parsers/requirement-blocks.ts:

• Added sectionPresence tracking to DeltaPlan interface to track whether each delta section was found
• Implemented getSectionCaseInsensitive() helper for case-insensitive section lookup
• Updated parseDeltaSpec() to use case-insensitive lookup and track section presence

Modified src/core/validation/validator.ts:

• Enhanced validateChangeDeltaSpecs() to detect empty delta sections and missing headers
• Added clear error messages for both scenarios to guide users
• Implemented formatSectionList() helper for user-friendly section name formatting

Added test in test/core/validation.test.ts:

• Test case verifying mixed-case delta headers are properly recognized

Test Plan

• Existing tests pass
• New test for case-insensitive headers added
• Manual testing with empty sections produces clear error messages
• Manual testing with missing headers produces appropriate guidance

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