Week 4 Phase 1A: solution-free starter surface + sync guards

[skyzh]

Why

Phase 1A of the Week 4 course delivery (task #201, under Cindy's task #200): audit and complete the solution-free learner starter against all nine feature APIs and the 7-day map, with fail-closed synchronization and anti-leakage controls. Stacked on the frozen reference stack (#258); base branch agent/week4v3-cow-radix-plan @ 563b114. Stack #259 is untouched.

What changed

• src/tiny_llm/agent/workspace.py — rewritten to the simplified reference surface (authorization core + effect receipts; no journal/undo/session binding, matching Day 2's simplified design).
• src/tiny_llm/agent/loop.py — rewritten to the Day-1 contract: duck-typed session recording, no context/control/session imports (matches the reference loop signature).
• src/tiny_llm/agent/generation.py — duck-typed cancellation so control stays a Day 6 module.
• src/tiny_llm/agent/receipts.py — added ReceiptStore.close.
• src/tiny_llm/agent/__init__.py — exports exactly the reference __all__ (52 names); removed stale old-course exports.
• Removed stale old-course starter modules: context.py, evaluation.py, recovery.py.
• tests_refsol/test_week_4_starter_sync.py (new) — fail-closed guards: starter/reference API parity per module, starter bodies are solution-free (pass/TODO only), the reference implements every starter member, feature/day map, __all__ parity, stale-module absence. 54 sync tests.

Review note

• 159/159 week-4 tests pass (105 reference + 54 sync); ruff check + format clean; weeks 1-3 unchanged (pre-existing unbuilt-extension failures only).
• No reference-solution, book, or existing-stack-head edits. Frozen for Sentinel's Phase 1B (book) and the gated Oracle/Sage/Tuner/Scholar reviews.

AI-Assisted: DeepSeek V4 Flash + Forge

[skyzh]

Superseded per Cindy's rolling day-by-day starter directive: task #201 is now Day-1-only first (from #243). The full-starter PR is replaced by the Day 1 starter successor.