chore: move compass out of docs/, which pkgdown deletes

compass.toml, JOURNAL.md, STATUS.md and decisions/ were sitting inside pkgdown's
output directory. Asked directly, pkgdown listed docs/pm and docs/decisions among
the 28 top-level entries clean_site() would delete, and the guard that would have
refused -- check_dest_is_pkgdown() -- was satisfied by docs/pkgdown.yml. After the
move it lists 26 and none of them are compass's.

Nothing was lost. The journal had no entries and there were no decision records
yet, so this was the cheapest moment to move.

The .gitignore workaround goes with it. Re-including two children of an excluded
docs/ forced the rule to be written as /docs/* plus two negations, which changed
the anchoring and made the fixture corpus's own docs/ need a separate rule. A bare
docs/ matches at any depth again, so both are unnecessary.

.Rbuildignore gains ^pm$ -- R CMD check flags a non-standard top-level directory.

Compass reads both layouts, so this repository worked either way; the point is
that docs/ is a directory another tool empties.

pm/JOURNAL.md

@@ -0,0 +1,14 @@
+# Project journal
+
+Append-only, newest first. **Entries are never edited** — the value of this file is
+that it records what was believed at the time, including the parts that turned out
+wrong. Where things stand *today* is in `STATUS.md`, which is generated.
+
+Four lines per entry. The analysis belongs in the issue or the decision record; this
+file carries the reasoning and the pointers.
+
+- **Why** — the driver. The one line git cannot reconstruct later.
+- **Obligates** — issues this change created elsewhere. Numbers, not prose.
+- **Refs** — commits, issues, decision records.
+
+---