Three small UX wins from the post-v0.2.0 review queue. Stacked on
ux-polish-2 (#20).

#5 — Filter the Findings tree
- Pipeline-Check: Filter Findings command opens an InputBox; matches
  against rule ID, message body, and file path (case-insensitive
  substring). Re-invoke pre-fills the current filter so users can
  edit or clear (empty input clears).
- New `$(filter)` button on the Findings view title bar.
- FindingsTreeProvider gets getFilter/setFilter/applyFilter. The
  badge tracks the filtered count; `lastFindingUris` still tracks
  the unfiltered universe so a publish for a currently-hidden URI
  still wakes the tree up (otherwise a CLEAR of a filtered-out
  finding would never refresh).
- 8 tests covering: default empty, rule-id match, case-insensitivity,
  message-body match, fsPath match, clear via empty string,
  whitespace trim, badge reflects filtered count.

#4 — Open Finding (non-preview)
- Pipeline-Check: Open Finding context-menu entry on tree leaves.
  Opens with `preview: false` — pins the file as a permanent tab so
  triaging multiple findings side-by-side doesn't create tab
  clutter. The single-click default still uses preview, keeping the
  common "scan through findings" flow lightweight.
- LeafLike type widened to include `uri` and `diagnostic.range`.

#7 — Quieter clipboard confirmations
- Copy Rule ID and Copy LSP Install Command switch from
  showInformationMessage (modal toast) to setStatusBarMessage with
  a 2-second TTL (CONFIRM_TTL_MS). The copy succeeds silently 95%
  of the time anyway; the status bar confirmation reads without
  stealing focus.

Wiring
- package.json: three new command declarations; view/title button
  for filter (group navigation@1, between scan and changeGrouping);
  view/item/context entry for openNonPreview (group navigation@0,
  top of the menu); commandPalette gates the leaf-context commands
  to `false` so they don't show palette-wide without a node.
- src/findingsView.ts: filter state + setFilter + getFilter +
  applyFilter (in findings()); sets the `pipelineCheck.filterActive`
  context key so future manifest contributions can paint a "filter
  active" affordance.
- src/test/integration/activation.test.ts: command-registration
  expected list grows by filter + openNonPreview.

Total: 137 tests pass (was 129 on #20; +8 filter behaviour). Lint,
compile, smoke, integration-compile all green.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>