Give every fact one home, and stop the README restating values

[robertblust]

Spotted while reading the two side by side: the README's How matching decides section had drifted into restating docs/matching.md. The five signal weights, the score cap, the auto-merge threshold, and both algorithm names appeared in both files.

Changing one weight meant editing three places — FuzzyMatcher, matching.md, and a README paragraph nobody would think to check.

What changed

README keeps the shape, drops every value. It still carries the three confidence layers, the blocking-finds / scoring-grades distinction, and the property worth knowing before anything else — that automatic fuzzy merging ships off. It now says plainly that docs/matching.md is the single place those values are defined.

A weight change now touches two files, and the doc is versioned per matcher version, so that change is an append rather than an edit.

CLAUDE.md gains the rule that would have prevented it. Not "don't duplicate" — too vague to act on — but:

• an ownership table naming the one file that owns each kind of fact (code / matching.md / specs / roadmap-notes.md / contracts / README)
• the edit test: before writing a number, threshold, or algorithm name into prose, ask if this changes, how many files must I touch? More than one → link instead

Plus two rules that already bit us:

• Specs are frozen at merge. Slice 3 amended R4-1 in roadmap-notes.md rather than rewriting slice 2's spec. That was already the practice; it was never written down.
• A second repository links, never restates. The org profile drifted to "Core in development" while two slices had shipped, because it restated a roadmap living here. No CI in one repo can catch that.

Documentation only — no code, no test changes.

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