Per-workspace .cdidxignore not loaded from .codeindex config directory

[Widthdom]

Summary

Currently, .cdidxignore patterns are only recognized in the indexed project root and inherited down to child directories (FileIndexer line 67, IgnoreFileNames = [".gitignore", ".cdidxignore"]). Monorepo and multi-workspace setups have no way to define workspace-local ignore rules in a central .codeindex/config/ directory without modifying the root .cdidxignore or .gitignore. This blocks scenarios like: "Index /workspace/repo1 AND /workspace/repo2 from a shared .codeindex/workspaces/ config manifest."

Where

• src/CodeIndex/Indexer/Scanning/FileIndexer.cs:67 (IgnoreFileNames hardcoded, no config-dir variant)
• src/CodeIndex/Indexer/Scanning/FileIndexer.cs:1526 (LoadIgnoreRulesForDirectory iterates IgnoreFileNames; no config-dir lookup)
• src/CodeIndex/Cli/IndexCommandRunner.cs:95 (no per-workspace config loading before FileIndexer construction)

Suggested approach

1. Add support for a .codeindex/.cdidxignore file (workspace-scoped) separate from project-root .cdidxignore
2. Modify LoadIgnoreRulesForDirectory to check for this workspace-root file once at initialization
3. Create a new IgnoreRulesForWorkspace class that merges workspace-root, ancestor, and directory-level rules in the correct precedence
4. Pass the workspace ignore root to FileIndexer via constructor (already supports ignoreRuleRoot parameter)
5. Document the new .codeindex/.cdidxignore in USER_GUIDE with examples for multi-workspace setups
6. Store the resolved workspace ignore ruleset in indexing metadata for future audits

[Widthdom]

Work started on branch fix-issue2023-2022-1832-1947.