chore(shared): add EXCLUDED_DIRS constant for recursive FS scans

Introduces a frozen Set<string> of directory basenames (node_modules, .git,
dist, build, .next, .sisyphus, .omx, .turbo, coverage, out, .cache,
.vscode-test, target, .local-ignore) that callers performing recursive
filesystem scans should skip.

This is shared infrastructure for upcoming fixes in rules-injector,
command-discovery, and claude-code-command-loader that currently descend
into node_modules and other junk directories, causing slow plugin init
and slow edit loops when the plugin is launched in-tree.

src/shared/index.ts

@@ -79,3 +79,4 @@ export * from "./log-legacy-plugin-startup-warning"
 export * from "./task-system-enabled"
 export * from "./parse-tools-config"
 export { parseModelString } from "./model-string-parser"
+export { EXCLUDED_DIRS } from "./excluded-dirs"