feat(#1997): add prettier markdown formatter to treefmt

- .prettierrc: proseWrap=preserve (no prose reflow)
- .prettierignore: exclude hivectl-cli.md (auto-generated) + 11 docs
  with multi-line list-item continuations prettier would strip to col 0
  (CommonMark limitation in prettier's list handling)
- format 16 markdown files: cosmetic only (*→_, table alignment,
  heading normalisation) — verified no broken continuations, idempotent

.prettierignore

@@ -2,3 +2,19 @@
 # The hivectl-docs CI check diffs this against fresh binary output,
 # so reformatting it would break that check.
 docs/tools/hivectl-cli.md
+
+# Files with multi-line list-item continuations that prettier strips to col 0.
+# prettier's `proseWrap: "preserve"` prevents prose reflow but not list-item
+# indentation normalisation, so these files need manual re-wrap before
+# adding them back to the formatter scope.
+CLAUDE.md
+docs/approvals.md
+docs/conventions.md
+docs/gateway.md
+docs/matrix.md
+docs/persistence.md
+docs/terminal-rendering.md
+docs/tools/matrix.md
+docs/turn-loop.md
+docs/web-ui/agent.md
+docs/web-ui/dashboard.md