ci(incremental): merge fitted experimental BKT params instead of copying default

[mjyang00001]

Companion to #118, for the second workflow that runs on the same cron.

incremental-content-update.yml ("Automated Content Update (Partial)") ends its "Move and prepare files" step with cp defaultBKTParams.json experimentalBKTParams.json, which makes the two A/B slots byte-identical and discards any fitted values in the experimental slot. This replaces that copy with bkt/merge_experimental.py from the already-cloned OATutor-Tooling checkout: for each KC in the freshly generated default file it takes the fitted value where one exists and falls back to default's own value otherwise, so experimental tracks default's live KC set (new KCs added, removed/renamed KCs dropped) while keeping tuned values.

Safe on the incremental path specifically because the "Prepare for incremental update" step (added in #110) restores defaultBKTParams.json back to bktParams.json before final.py runs, so by the time the merge executes, default holds the full KC set rather than only the sheets that changed in that run.

Depends on CAHLR/OATutor-Tooling#16, which adds bkt/merge_experimental.py and bkt/output/experimentalBKTParams.json. The workflow clones tooling's default branch, so that PR must merge first or the step will fail.

Test plan

• Merge Feature/bkt fitting OATutor-Tooling#16 first
• workflow_dispatch this workflow and confirm the merge step reports the fitted-KC count
• Verify experimentalBKTParams.json on content-staging differs from defaultBKTParams.json and has the same KC set
• No "BKT Parameter does not exist" errors in the browser console

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https://claude.ai/code/session_014Uq7T4wTcGUWnMAbKyPHQ3