Add tests for untested helpers: marketplace tree filtering, contributors, storage/debounce hooks

[Rinava]

Problem

Only the pure parseGitHubUrl from src/lib/skill/marketplace.ts has tests. Untested but load-bearing:

• discoverSkills tree filtering (marketplace.ts:253) — path-prefix logic over untrusted GitHub tree responses.
• importSkillDir's MAX_EXTRA_FILES / MAX_FILE_BYTES caps — the guards themselves are unverified.
• src/lib/contributors.ts — sizedAvatar (pure) and the bot-filtering in fetchContributors.
• src/lib/use-local-storage.ts / src/lib/use-debounced-value.ts — JSON-parse fallbacks and timer logic.

This is exactly the kind of code that regresses silently.

Implementation notes

• Marketplace + contributors are node-testable today with a mocked fetch — see the existing tests in src/lib/skill/__tests__/ for the house style.
• The two hooks need the jsdom setup tracked separately — fine to leave them for a follow-up PR.

Good first issue because… these are functions with clear inputs and outputs, and the existing lib tests show exactly what a good test file here looks like.

[Shreyaa0807]

/claim

[Rinava]

@Shreyaa0807 assigned you this one too since you claimed both. One heads-up before you start: the marketplace-tree and contributors helpers are plain functions — testable today in the existing node environment — but the storage/debounce hook tests need the jsdom setup tracked in #124, which nobody has picked up yet. Totally fine to start with the pure parts and leave the hooks for later (or grab #124 too if you're feeling ambitious). No rush on any of it.