[action-translation] Wire zh-cn sync automation and reconcile drift (Phase 0, Track B)

Part of Phase 0 tracked in QuantEcon/action-translation#74. This repo is the English source for lecture-python.zh-cn, which holds 79 of 121 current lectures but has been dormant since 2025-11-26 — 40+ lecture PRs here (including the Kalman rewrites, Hansen–Singleton, unemployment dynamics, and the LQ permanent-income trilogy) have not flowed through, and 8 translated files still sit under pre-rename filenames.

Scope note: Phase 0 only reconciles the 79 already-translated lectures and turns automation on. Bulk translation of the 42 missing lectures and the restart of the RA review campaign are Phase 2 of the rollout (see QuantEcon/action-translation#74).

Todo

1. Repair the target repo so sync PRs can land correctly

• Rename the 8 orphaned files in lecture-python.zh-cn to match current source names (optgrowth.md → os.md, optgrowth_fast.md → os_numerical.md(?), cake_eating_problem.md/cake_eating_numerical.md, coleman_policy_iter.md, egm_policy_iter.md, ifp.md, mccall_correlated.md → verify each against the source rename history before moving)
• Regenerate heading-maps after the renames (translate headingmap)
• Clear the two stale review PRs QuantEcon/lecture-python.zh-cn#63 and QuantEcon/lecture-python.zh-cn#87 (open since Sep/Oct 2025) — finish review and merge, or close, so the repo starts clean

2. Reconcile drift on the 79 translated lectures

• Generate an onboarding report against current HEADs (translate status / onboarding)
• Work through per-file drift decisions (source→target RESYNC vs backport vs manual); RESYNC drifted files with translate forward

3. Wire the automation (this repo)

• Add .github/workflows/sync-translations-zh-cn.yml, copied from lecture-python-programming with target-repo: QuantEcon/lecture-python.zh-cn (pin QuantEcon/action-translation@v0.15.0; include the \translate-resync comment trigger). Note: sync PRs for the 42 untranslated lectures will create new files — confirm desired behaviour with the action's docs-folder settings, or scope the workflow to existing files until Phase 2
• Confirm TRANSLATION_PAT and ANTHROPIC_API_KEY secrets are set on this repo

4. Review workflow (target repo)

• Add review-translations.yml and rebase-translations.yml to lecture-python.zh-cn, copied from lecture-python-programming.zh-cn (the target repo has a parked _future/ workflows folder — reconcile with what is there)

5. Verify end-to-end

• Trigger one sync on an already-translated lecture and confirm a translated PR opens, passes AI review, and builds in the target repo
• Tick off Track B in QuantEcon/action-translation#74

Start with QuantEcon/action-translation#74 and compare the workflows in lecture-python-programming and lecture-python-programming.zh-cn. Inspect the target repo's _future/ workflows, the eight renamed files, and the action's docs-folder settings before editing. Done means drift is reconciled, stale PRs are resolved, workflows and secrets are confirmed, and an end-to-end sync PR builds successfully.