docs: sync live generated theorem path count to thirteen

[AkiraTamai]

Summary

docs-sync only. docs/ROADMAP.md, docs/ARCHITECTURE.md and docs/LEAN_TRANSLATOR_SPEC.md already describe thirteen live generated theorem paths (through predicate_guard_collapse), but docs/LEAN_HARNESS_CONTRACT.md and .agents/skills/testing-mumei-lean-live-generated/SKILL.md still said eleven and stopped at ff_mul_associative. The implementation (tests/test_lean_bridge_e2e.py + the 13 tests/fixtures/*.proof-cert.json live paths) is the single source of truth, so the docs are moved to match it:

• docs/LEAN_HARNESS_CONTRACT.md: eleven → thirteen, plus per-path entries for ff_mul_add_distributive (12th, tactic search adopts mumei_ff_mod, Generated.Std.Algebra.Finite_field.ff_mul_add_distributive_correct) and predicate_guard_collapse (13th, widened ladder adopts tauto, Generated.Std.Core_predicates.predicate_guard_collapse_correct). Both are tactic-only adoptions, so bridge_lemma_hash is unchanged.
• .agents/skills/.../SKILL.md: heading (11 total) → (13 total) and list items 12/13.
• docs/CHANGELOG.md: new entry recording the two previously unlogged paths and the count sync.

No code, no vocabulary, and no contract constants change: translator_version = mumei-lean-translator-ir-v2 and bridge_lemma_hash = ee8cd3ba…4347 still match scripts/export_cert.py / scripts/expr_translator.py.

Regression commands

# mumei-lean
PYTHONPATH=scripts MUMEI_LEAN_SKIP_LIVE=1 python -m pytest tests/test_contract_vocabulary.py -q   # 6 passed
PYTHONPATH=scripts MUMEI_LEAN_SKIP_LIVE=1 python -m pytest -q                                    # 306 passed, 11 skipped

# cross-repo docs-sync gates (unchanged, run green on develop)
# mumei:       python3 scripts/check_contract_vocabulary.py
# mumei-agent: uv run pytest tests/test_contract_vocabulary.py -q

Link to Devin session: https://app.devin.ai/sessions/adfa18cdf78948c09b218b54aa5a83b0
Requested by: @AkiraTamai

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✅ Devin Review: No Issues Found

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