Scaffold CMOS 18 reformatter: harness, linter, parser, formatter, CLI

Initial phase-1 baseline of the karpathy/autoresearch-style loop.
The formatter module is the inner-loop artifact; parser and linter
are infra. The linter carries a LINTER_VERSION hash (v0.2.0) that
will force a re-baseline on any rule change.

Components:
- harness/diff.py: case-sensitive field-level substring diff
- harness/score.py: three-axis scoring (field, linter, canary exact)
- src/cmos/linter.py: 9 CMOS 18 structural rules, each Purdue/CMOS cited
- src/cmos/parser.py: locate ## Bibliography section, split entries
- src/cmos/formatter.py: prompt + OpenAI call with caller injection
- src/cmos/cli.py: cmos format path/to/draft.md
- scripts/run_loop.py: loop runner with --fake mode for no-API runs
- exemplars/: 3 canary seed exemplars (book, journal w/DOI, web),
  sourced from chicagomanualofstyle.org quick guide

Tests: 48 passing. Fake-mode baseline scalar = 0.000 on the 3 seed
exemplars (identity caller fails the linter on every rule). This is
the floor the real GPT-5 formatter needs to improve from.

tests/test_formatter.py

@@ -0,0 +1,83 @@
+"""Tests for src/cmos/formatter.py — the inner-loop iterable artifact.
+
+Because `formatter.py` is edited every iteration of the dev-time autoresearch
+loop, these tests intentionally test STABLE pieces: the prompt skeleton, the
+caller-injection seam, and a smoke check that the system prompt mentions key
+CMOS 18 rules. The actual LLM output shape is tested via the harness (running
+all exemplars through `score`) rather than pinned here — exact string
+matches on LLM output belong in the canary exact-match rate, not in unit
+tests.
+
+No real API calls in this file. Tests that hit the OpenAI API live in
+`tests/test_formatter_integration.py` (not yet created) and are gated on an
+OPENAI_API_KEY being set.
+"""
+
+from cmos.formatter import (
+    MODEL,
+    SYSTEM_PROMPT,
+    build_user_message,
+    format_bibliography_entry,
+)
+
+
+def test_default_model_is_gpt5():
+    # The user specified "GPT-5 / frontier reasoning" as the default. Override
+    # via the OPENAI_MODEL env var if gpt-5 is unavailable in your account.
+    assert MODEL == "gpt-5"
+
+
+def test_system_prompt_mentions_cmos_18():
+    assert "CMOS" in SYSTEM_PROMPT or "Chicago Manual of Style" in SYSTEM_PROMPT
+    assert "18" in SYSTEM_PROMPT
+
+
+def test_system_prompt_mentions_no_place_of_publication():
+    # CMOS 14.30 / 18th ed. change. The formatter MUST know this.
+    lower = SYSTEM_PROMPT.lower()
+    assert "place of publication" in lower
+
+
+def test_system_prompt_mentions_doi_preference():
+    assert "doi" in SYSTEM_PROMPT.lower()
+
+
+def test_system_prompt_mentions_italic_markers():
+    # Plan v1 uses Markdown `*Title*` for italics.
+    assert "*" in SYSTEM_PROMPT and "italic" in SYSTEM_PROMPT.lower()
+
+
+def test_build_user_message_contains_the_messy_input():
+    msg = build_user_message("yu, charles. interior chinatown. 2020")
+    assert "yu, charles. interior chinatown. 2020" in msg
+
+
+def test_formatter_uses_injected_caller():
+    """The formatter accepts a caller shim so tests (and the harness) can
+    substitute a fake OpenAI call. This is how the whole pipeline can run
+    without an API key during tests."""
+    recorded: dict = {}
+
+    def fake_caller(system: str, user: str) -> str:
+        recorded["system"] = system
+        recorded["user"] = user
+        return "Yu, Charles. *Interior Chinatown*. Pantheon Books, 2020."
+
+    output = format_bibliography_entry(
+        "yu, charles. interior chinatown. New York: Pantheon Books, 2020.",
+        caller=fake_caller,
+    )
+    assert output == "Yu, Charles. *Interior Chinatown*. Pantheon Books, 2020."
+    assert recorded["system"] == SYSTEM_PROMPT
+    assert "yu, charles" in recorded["user"]
+
+
+def test_formatter_strips_whitespace_from_caller_output():
+    # Language-model output often has leading/trailing whitespace or
+    # surrounding code fences. v0 only strips whitespace; code-fence
+    # stripping can be added when an exemplar forces it.
+    def fake_caller(system: str, user: str) -> str:
+        return "  Yu, Charles. *Interior Chinatown*. Pantheon Books, 2020.  \n"
+
+    output = format_bibliography_entry("anything", caller=fake_caller)
+    assert output == "Yu, Charles. *Interior Chinatown*. Pantheon Books, 2020."