feat(cli): add watch mode

[macayu17]

Fixes #458.

Summary

Adds a --watch flag for continuous local validation. It runs a full validation pass on startup, then listens for file create/write/ rename events and revalidates the changed file when it still matches the existing finder filters.

The watcher uses fsnotify, adds watches recursively for the configured search paths, picks up new directories as they are created, and exits cleanly on Ctrl+C.

Dependency note

fsnotify is used for cross-platform file watching, as requested in the issue.

Verification

• go test ./cmd/validator -run 'Test_getFlagsWatchValue|TestRunWatch|Test_emptyBoolEnvVarNoParseError' -count=1
• go build -o $env:TEMP\validator-watch-test.exe cmd\validator\validator.go
• Linux container CI path: go generate, go vet, gofmt, go build, go test -cover, nested justfile tests
• Coverage: 91.3%
• golangci-lint run ./... --timeout=10m
• golangci-lint run ./... --timeout=10m in pkg/validator/justfile
• git diff --check
• git diff --exit-code pkg/filetype/known_files_gen.go

[Copilot]

Pull request overview

Note

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

Adds a --watch (and CFV_WATCH env var) mode that performs an initial validation pass and then revalidates files when changes are detected via fsnotify.

Changes:

• New --watch/CFV_WATCH flag plumbed through validatorConfig/resolvedConfig and wired into mainInit.
• Implements a watchRunner using fsnotify with recursive directory watching and a static-file finder for per-event revalidation.
• Adds unit tests, docs (CLI flags, env vars, README), and changelog entry; adds fsnotify dependency.

Reviewed changes

Copilot reviewed 8 out of 9 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
cmd/validator/validator.go	Adds watch flag, runWatch/watchRunner implementation, and buildCLIWithFinder helper.
cmd/validator/watch_test.go	New tests for initial pass + change re-validation and finder-filtered events.
cmd/validator/validator_test.go	Adds --watch flag parsing tests and includes CFV_WATCH in empty-env-var test.
go.mod / go.sum	Adds github.com/fsnotify/fsnotify v1.10.1 dependency.
website/docs/reference/cli-flags.md	Documents the new -watch flag.
website/docs/reference/environment-variables.md	Documents the new CFV_WATCH env var.
README.md	Mentions watch mode in features list.
CHANGELOG.md	Adds entry for --watch mode (closes #458).

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