Do not keep a harness report in the library repository (#1021)

`propcheck.md` sits in the repository root. It is the output of `propcheck`,
one of the analysis harnesses that lives outside this repository, and it
arrived here because that harness wrote its report into the working directory
rather than resolving a path from its assembly. Every run since has been
invoked from elsewhere, so the file has not been touched by a run since it was
committed and says the corpus is 151 expressions and 1340 checks whether or not
that is still true.

A report nobody regenerates where they read it is worse than no report: it
carries a number with no way to tell how old the number is. The figures that
belong in this repository are the ones in
`Docs/Contributing/SimplificationContract.md`, which quotes them in prose next
to what they mean and is edited by hand when they move.

The `.gitignore` entries stop the same thing happening again for propcheck and
for the eleven other harnesses that write a report with the same shape, and
ignore the worktree directory the agents branch into, which is a full checkout
per branch and must never be added.

Author: Rafael-SOWNet

.gitignore

@@ -58,3 +58,20 @@ TestResults
 **/.ionide/**
 Sources/msbuild.binlog
 **/publish-output/**
+
+# Reports the out-of-tree analysis harnesses write beside the repository root.
+propcheck.md
+boundcheck.md
+canoncheck.md
+confluence.md
+comparison.md
+rulecheck.md
+simpsweep.md
+rootcheck.md
+crashcheck.md
+docsamples.md
+sympyparity.md
+egraph.md
+
+# Agent worktrees and local tool state.
+.claude/