Support .yfmignore to exclude files from processing

[martyanovandrey]

Description

Add support for a .yfmignore file in the project root to specify files and directories that should be skipped by the CLI. The file should accept relative paths and glob patterns.

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Use Case

• Exclude drafts, temporary files, large asset folders, or generated content from parse/validate/build steps.
• Speed up builds and reduce noise.

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Expected Behavior

• If .yfmignore exists, the CLI excludes matching files from all processing stages (parse, validate, build, search index, etc.).
• Patterns are resolved relative to the project root.

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Supported Syntax

• Glob patterns: *, **, ?, [abc]
• Directory patterns: directory with trailing slash
• Negation: ! to re-include files/folders
• Comments: lines starting with #
• Blank lines: are ignored
• Cross-platform: works consistently on all operating systems

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Example .yfmignore

# ignore drafts and tmp
drafts/**
tmp/**

# ignore images but keep logos
assets/images/**
!assets/images/logos/**

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Acceptance Criteria

• The CLI reads and applies .yfmignore when present.
• Files matching ignore patterns are not processed or reported as missing.
• Negation patterns work as expected.
• No change in behavior when .yfmignore is absent.
• Clear documentation is added, including examples.