fix: wire --poll flag through to chokidar in theme dev

[graygilmore]

Summary

• The --poll flag was defined on theme dev but never wired through to chokidar's usePolling option, making it a no-op
• On Linux/Windows, chokidar's fs.watch backend compares mtimeMs and suppresses change events when the timestamp hasn't changed. Build tools (like Gulp + Sass) that preserve the original source file's timestamp on compiled output cause file change events to be silently dropped
• This wires poll through the full chain: command → service → mountThemeFileSystem → chokidar.watch. When --poll is set, both usePolling: true and useFsEvents: false are passed to chokidar (useFsEvents: false is required because on macOS chokidar prefers FSEvents over polling even when usePolling: true is set)
• Also unhides the --poll flag and improves its description to tell users when to use it

Related: https://community.shopify.dev/t/theme-dev-sometimes-misses-file-updates/29269/4

Test plan

• New tests verify usePolling: true + useFsEvents: false are passed to chokidar when poll: true
• New tests verify usePolling and useFsEvents are absent from chokidar options when poll is not set
• All 44 existing tests in theme-fs.test.ts continue to pass
• Manual: shopify theme dev --poll enables chokidar polling mode
• Manual: shopify theme dev --help shows the --poll flag with improved description

🤖 Generated with Claude Code

[github-actions]

Differences in type declarations

We detected differences in the type declarations generated by Typescript for this branch compared to the baseline ('main' branch). Please, review them to ensure they are backward-compatible. Here are some important things to keep in mind:

• Some seemingly private modules might be re-exported through public modules.
• If the branch is behind main you might see odd diffs, rebase main into this branch.

New type declarations

We found no new type declarations in this PR

Existing type declarations

packages/cli-kit/dist/public/node/themes/types.d.ts

@@ -30,6 +30,7 @@ export interface ThemeFileSystemOptions {
     listing?: string;
     noDelete?: boolean;
     notify?: string;
+    poll?: boolean;
 }
 /**
  * Represents a theme on the file system.

[github-actions]

Coverage report

St.❔	Category	Percentage	Covered / Total
🟡	Statements	77.31%	14586/18868
🟡	Branches	70.9%	7238/10209
🟡	Functions	76.3%	3706/4857
🟡	Lines	78.8%	13790/17501

Test suite run success

3813 tests passing in 1473 suites.

Report generated by 🧪jest coverage report action from d4fdcb5