ci: add dead code detection with knip, ts-prune, and ts-unused-exports

[DeJeune]

What this PR does

Before this PR:
No automated dead code detection in CI. Unused files, exports, and dependencies accumulate silently.

After this PR:
Three dead code detection tools run as a parallel CI job on code changes:

• knip — detects unused files, dependencies, and exports
• ts-prune (via pnpm dlx) — detects unused TypeScript exports
• ts-unused-exports (via pnpm dlx) — detects unused exports across tsconfig projects

All three tools run in strict/error mode — CI fails if any dead code is detected.

Why we need it and why it was done in this way

The following tradeoffs were made:

• ts-prune and ts-unused-exports use pnpm dlx to avoid adding devDependencies; knip is installed because it needs a typed config file
• knip config includes ignoreDependencies for false positives: dynamic imports (@cherrystudio/embedjs-loader-*), build plugins (@swc/plugin-styled-components, @electron/notarize), peer deps (TipTap collaboration, tailwindcss, etc.)
• The deadcode-check CI job runs in parallel with existing jobs, gated by the same changes filter (main/renderer/shared)

The following alternatives were considered:

• Single tool only — each tool catches different categories, using all three gives broader coverage
• Warning-only mode — rejected in favor of strict enforcement to prevent dead code accumulation

Breaking changes

None

Special notes for your reviewer

• All three tools run in error mode: CI will fail if dead code is detected
• knip config ignores ambient .d.ts files, test fixtures, scripts, and dynamically-imported peer dependencies to reduce false positives
• Existing dead code needs to be cleaned up before this CI check passes

Checklist

• PR: The PR description is expressive enough and will help future contributors
• Code: Write code that humans can understand and Keep it simple
• Refactor: You have left the code cleaner than you found it (Boy Scout Rule)
• Upgrade: Impact of this change on upgrade flows was considered and addressed if required
• Documentation: A user-guide update was considered and is present (link) or not required.
• Self-review: I have reviewed my own code before requesting review from others

Release note

NONE

[EurFelux]

Note

This comment was translated by Claude.

In #13531, I previously used knip and found some false positives. However, I didn't examine the configuration closely but rather had cc verify them individually. For automation, the configuration needs to be confirmed again.

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Original Content

#13531 之前用 knip 发现会有一些误报，不过我没仔细看配置，而是让cc去逐个验证了。要自动化的话，需要再确认下配置