fix: lint newly created files in dev watchers

[charan-rathore]

Description

Addresses the new-file reporting part of #807.

ESLint, Stylelint, and Biome currently listen for file changes and deletion, but not creation. Chokidar sends an add event when a file is created, so a new file with a lint error gets no diagnostics until it is saved again.

Send add events through the same handler as change. Set ignoreInitial: true because each checker already runs an initial full check before creating its watcher.

The regression tests use the built checker workers, real linters, and real filesystem events. They first verify that the watcher handles an existing file, then create another file once and require diagnostics for that new path. All three tests fail before this change and pass after it.

Validation: all 44 unit tests, package typecheck, production build, and changed-file Biome checks pass. Repository-wide lint already fails on main because an untouched utils.spec.ts needs formatting; this patch leaves it unchanged.

This does not address the separate stale-diagnostics/deletion race described in #807.

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commit: b3c05a2

[charan-rathore]

Closing since this has sat without activity. Glad to reopen if someone wants to take it forward.