fix: implement NOT operator with LINQ post-filtering

[dluc]

Summary

Fixes the NOT operator issues - the NOT operator now works correctly using a hybrid FTS + LINQ approach.

Problem

The NOT operator had two problems:

1. Standalone NOT crashed: km search "NOT foo" threw FTS5 syntax error
2. NOT didn't exclude: km search "foo AND NOT bar" returned documents containing both

Solution

Hybrid FTS + LINQ filtering:

• FTS5 handles positive terms only (no NOT terms sent to FTS)
• NOT terms are collected separately and excluded via LINQ post-filtering
• Standalone NOT queries work by fetching all documents then filtering

Changes

• src/Core/Search/NodeSearchService.cs:

  • Added FtsQueryResult record to separate FTS query from NOT terms
  • Modified FtsQueryExtractor to collect NOT terms separately
  • Added ApplyNotTermFiltering() for LINQ exclusion

• src/Core/Search/SqliteFtsIndex.cs:

  • Added GetAllDocumentsAsync() for standalone NOT queries

• src/Core/Search/Query/Parsers/InfixQueryParser.cs:

  • Added single-quote support (was treating 'AND' as literal with quotes)

• src/Main/CLI/Commands/ExamplesCommand.cs:

  • Added NOT operator examples (infix and JSON format)

• tests/Core.Tests/Search/SearchEndToEndTests.cs:

  • Added 9 E2E tests for NOT operator scenarios

• KNOWN-ISSUES.md: Cleaned up, only open issues remain

Usage

# Standalone NOT - find documents NOT containing "secret"
km search "NOT secret"

# Exclusion - find "foo" but NOT "bar"
km search "foo AND NOT bar"

# JSON format
km search '{"content": {"$not": "deprecated"}}'
km search '{"$nor": [{"content": "alpha"}, {"content": "beta"}]}'

Note: Use explicit AND between terms (e.g., foo AND NOT bar)

Test plan

• All 546 tests pass (332 Core + 214 Main)
• Zero skipped tests
• build.sh passes with 0 warnings
• format.sh passes