feat: add --ignore-file filtering

[macayu17]

Closes #457.

Summary

Adds a repeatable --ignore-file flag for gitignore-style pattern files such as .dockerignore and .prettierignore. Each file is resolved relative to the search path root, missing files are skipped, and multiple ignore files are additive.

This also wires the same behavior through ignore-files in .cfv.toml and the comma-separated CFV_IGNORE_FILES environment variable. The new behavior is covered in finder unit tests and CLI testscript cases, with docs updated for the new flag, config key, and env var.

Verification

• go test ./pkg/finder ./pkg/configfile ./cmd/validator -run "Test_fsFinderGitignore|Test_fsFinderIgnoreFiles| Test_fsFinderMultipleIgnoreFiles|TestLoadValid|Test_ignoreFilesEnvVar|Test_getFlags|TestScript/ignore_file" -count=1
• Full local pipeline in a Linux container: go vet, gofmt, golangci-lint, go generate diff check, Linux
  build, go test -cover ./..., nested justfile tests
• Coverage: 93.3%

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Adds support for applying additional gitignore-style ignore files during file discovery, configurable via CLI flag, config file key, and environment variable.

Changes:

• Introduces --ignore-file (repeatable) and ignore-files config key, plus CFV_IGNORE_FILES env var support.
• Extends filesystem finder to load ignore patterns from explicit ignore files (even when not in a Git repo).
• Updates docs, schema, changelog, and adds unit/integration tests covering the new behavior.

Reviewed changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
website/docs/reference/environment-variables.md	Documents CFV_IGNORE_FILES mapping and format.
website/docs/reference/configuration-keys.md	Adds ignore-files config key to the reference table.
website/docs/reference/cli-flags.md	Adds -ignore-file to CLI flags reference.
website/docs/guides/filtering-exclusion.md	Adds a guide section explaining ignore files and updates evaluation order.
website/docs/guides/configuration-file.md	Adds ignore-files to examples and key table.
pkg/finder/fsfinder.go	Implements loading ignore patterns from explicit ignore files.
pkg/finder/finder_test.go	Adds unit tests for ignore-file behavior.
pkg/configfile/schema.json	Adds JSON schema entry for ignore-files.
pkg/configfile/configfile_test.go	Adds config parsing test for ignore-files.
pkg/configfile/configfile.go	Adds IgnoreFiles field to config struct.
cmd/validator/validator_test.go	Adds tests for --ignore-file flags and CFV_IGNORE_FILES.
cmd/validator/validator.go	Adds --ignore-file, env var parsing, config-file application, and wiring into finder opts.
cmd/validator/testdata/ignore_file.txtar	Adds integration-style test scenarios for ignore files.
CHANGELOG.md	Notes the new --ignore-file option.

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