feat: add kit-parity skill to track upstream SvelteKit changes

[SomaticIT]

What

Adds a kit-parity Claude Code skill (.claude/skills/kit-parity/) that keeps the library in sync with SvelteKit's remote-functions API over time, enforcing the CLAUDE.md parity rule: behavior must match kit unless documented in DIFFERENCES.md.

How it works

The skill walks through six gated steps:

1. Detect — fetch-upstream.sh check downloads the 26 tracked upstream files (runtime sources, spec files, form-utils, Remote* types, the remote-functions docs page) from raw.githubusercontent.com and compares content hashes against baseline.json (currently kit 2.69.3). Content hashes were chosen over commit SHAs because they only flag files that actually matter, and raw.githubusercontent.com remains reachable in sandboxed sessions where github.com/api.github.com are proxied. The kit CHANGELOG and docs provide the "why" for each change.
2. Classify — each behavioral delta is bucketed: transport/server-only (skip), covered by a documented difference (verify the doc still holds), portable behavior (implement), or unclear → stop and ask the user with options + recommendation. New divergences are never decided silently.
3. Diff the codebases — includes the full upstream↔local file mapping table, the list of local-only machinery (refresh_keys/refresh_all, mutation epoch, config, validation), and the port invariants (no this capture in resource factories, no unhandled rejections, .svelte.ts suffix).
4. Port relevant tests — upstream spec → local suite mapping table; transport-only tests are noted as skipped rather than ported.
5. Implement — with DIFFERENCES.md updated in the same commit for any divergence touched.
6. Verify & rebaseline — check/test/build/playground, then fetch-upstream.sh baseline records the new sync point.

Contents

• .claude/skills/kit-parity/SKILL.md — the workflow
• .claude/skills/kit-parity/fetch-upstream.sh — check / baseline / download modes (validated: clean check exits 0; a tampered baseline correctly reports CHANGED and exits 1)
• .claude/skills/kit-parity/baseline.json — initial baseline (kit 2.69.3, 26 file hashes)
• CLAUDE.md — pointer to the skill under the kit-parity rule

Verification

• Script tested in all three modes against live upstream, including a negative test (tampered hash → change detected).
• Repo remains green on the upgraded toolchain from main: svelte-check 0 errors/0 warnings, 103/103 tests, svelte-package + publint clean.

🤖 Generated with Claude Code

https://claude.ai/code/session_01EJj9ab3v9HjudgX8L1x821

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