feat(docs): "?" help affordance on every filter mount + docs loader fixes

The filter manual (1ed1602e) was reachable only by knowing /docs
already existed — nothing in the UI pointed at it (the parked manual
plan's open decision). Every filter-builder mount now carries a "?"
in its header linking to the guide (opens in a new tab so drawer and
editor state survive): the All Books drawer and the collection rules
page — the two mounts sharing FilterBuilderRows. The docs home
indexes the manual (guide, cookbook, generated field reference).

Two docs-loader fixes surfaced by pinning this:
- Folder URLs resolve to their index page: /docs/user/filters now
  serves user/filters/index.md (the manual is a folder: guide +
  cookbook + generated reference); previously only root /docs had
  index handling.
- The docs folder path is resolved by walking upward (bounded):
  `go test` runs test binaries from the PACKAGE directory, so the
  relative "docs" path resolved to a folder that didn't exist there
  and every /docs route 404'd in tests while working in prod —
  found by the new pin, fixed so tests exercise the docs routes
  exactly as prod serves them.

Pins: TestFilterHelpAffordance (folder URL serves the guide; drawer
and rules pages carry the link). internal/docs suite + full suite
green (239s).

cmd/server/tests/podcast_test.go

@@ -2520,3 +2520,60 @@ func TestFilterLabelsEndpoint(t *testing.T) {
 		assert.Equal(t, http.StatusBadRequest, code, "%s", string(body))
 	})
 }
+
+// ---- the "?" affordance: the builder links to its manual ----
+
+// TestFilterHelpAffordance pins the help links (2026-11-01): every
+// filter-builder mount carries a "?" linking to the manual, and the
+// manual's FOLDER URL resolves to its index page (the docs loader's
+// folder-index fallback — the guide is docs/user/filters/index.md).
+func TestFilterHelpAffordance(t *testing.T) {
+	setup := setupTestServer(t)
+	regToken := setup.RegularToken
+
+	t.Run("docs_folder_url_serves_the_guide", func(t *testing.T) {
+		req, _ := http.NewRequest("GET", setup.Server.URL+"/docs/user/filters", nil)
+		resp, err := http.DefaultClient.Do(req)
+		require.NoError(t, err)
+		body, _ := io.ReadAll(resp.Body)
+		_ = resp.Body.Close()
+		require.Equal(t, http.StatusOK, resp.StatusCode, "the folder URL resolves to index.md; body: %.400s", string(body))
+		assert.Contains(t, string(body), "Filters — the complete guide")
+	})
+
+	t.Run("drawer_carries_help_link", func(t *testing.T) {
+		req, _ := http.NewRequest("GET", setup.Server.URL+"/bookshelf", nil)
+		req.Header.Set("Authorization", "Bearer "+regToken)
+		resp, err := http.DefaultClient.Do(req)
+		require.NoError(t, err)
+		body, _ := io.ReadAll(resp.Body)
+		_ = resp.Body.Close()
+		require.Equal(t, http.StatusOK, resp.StatusCode)
+		assert.Contains(t, string(body), `href="/docs/user/filters"`,
+			"the All Books drawer links to the filter guide")
+	})
+
+	t.Run("rules_page_carries_help_link", func(t *testing.T) {
+		marker := fmt.Sprintf("helpaff%d", time.Now().UnixNano()%100000)
+		code, body := podcastAuthed(t, setup, regToken, "POST", "/api/collections",
+			`{"name":"test HelpAff `+marker+`","filter_tree":{"scope":"episodes","mode":"all","children":[]}}`)
+		require.Equal(t, http.StatusCreated, code, "%s", string(body))
+		var coll struct {
+			ID string `json:"id"`
+		}
+		require.NoError(t, json.Unmarshal(body, &coll))
+		t.Cleanup(func() {
+			podcastAuthed(t, setup, regToken, "DELETE", "/api/collections/"+coll.ID, "")
+		})
+
+		req, _ := http.NewRequest("GET", setup.Server.URL+"/collections/"+coll.ID+"/rules", nil)
+		req.Header.Set("Authorization", "Bearer "+regToken)
+		resp, err := http.DefaultClient.Do(req)
+		require.NoError(t, err)
+		pageBody, _ := io.ReadAll(resp.Body)
+		_ = resp.Body.Close()
+		require.Equal(t, http.StatusOK, resp.StatusCode)
+		assert.Contains(t, string(pageBody), `href="/docs/user/filters"`,
+			"the collection rules editor links to the filter guide")
+	})
+}