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.

exemplars/README.md

@@ -0,0 +1,30 @@
+# Exemplars
+
+Each `.toml` file is one exemplar: a messy input, a canonical structured
+record, and an expected CMOS 18 bibliography string.
+
+## Format
+
+```toml
+# IMPORTANT: all top-level keys (including expected_bibliography) must come
+# BEFORE any [section] header, or TOML will absorb them into that section.
+source = "Purdue OWL CMOS 18 NB PDF, p. X"   # provenance, required
+type = "book"                                # book | journal | chapter | web | ai | ...
+tags = ["book", "single-author"]
+canary = false                               # if true, exact-string match required
+messy_input = "..."
+expected_bibliography = "Last, First. *The Book Title*. Publisher Name, 2023."
+
+[canonical]
+# Structured fields for the field-level diff. Fields vary by type.
+author = "Last, First"
+title = "The Book Title"
+publisher = "Publisher Name"
+year = 2023
+```
+
+## Primary sources
+
+- Purdue OWL CMOS 18 NB PDF: https://owl.purdue.edu/owl/research_and_citation/chicago_manual_18th_edition/documents/cmos-nb-20250731.pdf
+- CMOS official quick guide: https://www.chicagomanualofstyle.org/tools_citationguide/citation-guide-1.html
+- Widener "New in 18th" LibGuide: https://widener.libguides.com/Chicago18/New