feat(phase 4f): per-item timeline_exclude flag (hide noise from /timeline)

m's stated use case: home VTODOs (shopping list) shouldn't pollute the
chronological /timeline by default, but they should stay visible on the
home detail page itself. This adds an item-level switch with four kinds
and a URL override to peek at everything when wanted.

## Schema (migration 0015)
- timeline_exclude text[] NOT NULL DEFAULT '{}'
- items_timeline_exclude_idx GIN
- items_unified view rebuilt to surface the new column
- Behaviour-neutral: empty array = unchanged from today. m flips the
  toggle himself via /admin/bulk or the detail-page form.

## Aggregation
- web/timeline.go: pre-compute the per-kind keep-list via keepFor(kind)
  before fanning out — items with the kind in their exclude array are
  dropped entirely (no CalDAV call wasted on excluded sources). Doc and
  creation rows check the per-item flag inline. `?include_excluded=1`
  (URL) and `include_excluded:true` (MCP arg) override the filter.
- store.Item.ExcludesTimelineKind(kind) helper accepts either singular
  ("todo") or plural ("todos") to bridge the kind-constant / persisted-
  value naming choice — see comment for the why.

## UI
- /i/{path} grows a "Timeline behaviour" collapsible section with four
  checkboxes (todos / events / docs / creation) and helper text. Open by
  default when any kind is excluded, so m can see at a glance what's
  hidden for this item.
- /admin/bulk gains a "timeline todos" select with "Exclude from timeline"
  and "Re-include on timeline" — the other three kinds stay editable
  per-item only per the task brief (most common use case is just todos).

## MCP
- update_item accepts timeline_exclude as a partial-update field with an
  enum-restricted whitelist; unknown values dropped silently.
- itemView always emits timeline_exclude (defaults to []) so consumers
  can render the toggle state without a second round-trip.

## Tests
- Migration + GIN index landed
- Item with timeline_exclude=['todos'] hides the VTODO from /timeline
- ?include_excluded=1 brings it back
- Bulk action toggles the array idempotently in both directions
- Detail page renders all 4 checkbox affordances

## docs/design.md
§12 gains a "Per-item exclusion" subsection documenting semantics, the
URL override, the bulk action, and the "detail page still shows everything"
invariant.

## Out of scope (per task brief)
- Per-tag exclusion (per-item is clearer)
- Per-day exclusion (overkill)
- Dashboard exclusion (m only flagged timeline; dashboard's "today" view
  should still show shopping today if it's due today)
- Auto-seeding home with timeline_exclude=['todos'] (m runs once himself
  via /admin/bulk after the deploy — schema change stays behaviour-neutral)

mcp/tools.go

@@ -82,7 +82,7 @@ func RegisterProjaxTools(s *Server, st *store.Store, tl TimelineBuilder) {
 	})
 	s.Register(Tool{
 		Name:        "update_item",
-		Description: "Partial update of an existing item. Pass any subset of title/slug/content_md/status/tags/management/parent_paths/pinned/archived and the Phase-4d public-listing fields (public, public_description, public_live_url, public_source_url, public_screenshots). parent_paths replaces the full parent list.",
+		Description: "Partial update of an existing item. Pass any subset of title/slug/content_md/status/tags/management/parent_paths/pinned/archived, the Phase-4d public-listing fields (public, public_description, public_live_url, public_source_url, public_screenshots), or the Phase-4f timeline_exclude array. parent_paths replaces the full parent list.",
 		InputSchema: json.RawMessage(`{
 		  "type": "object",
 		  "properties": {
@@ -101,7 +101,8 @@ func RegisterProjaxTools(s *Server, st *store.Store, tl TimelineBuilder) {
 		    "public_description":  {"type": "string"},
 		    "public_live_url":     {"type": "string"},
 		    "public_source_url":   {"type": "string"},
-		    "public_screenshots":  {"type": "array", "items": {"type": "string"}}
+		    "public_screenshots":  {"type": "array", "items": {"type": "string"}},
+		    "timeline_exclude":    {"type": "array", "items": {"type": "string", "enum": ["todos","events","docs","creation"]}, "description": "Phase 4f — kinds to hide from /timeline (per item)"}
 		  }
 		}`),
 		Handler: updateItemTool(st),
@@ -390,6 +391,8 @@ type itemView struct {
 	PublicLiveURL      string   `json:"public_live_url"`
 	PublicSourceURL    string   `json:"public_source_url"`
 	PublicScreenshots  []string `json:"public_screenshots"`
+	// Phase 4f
+	TimelineExclude    []string `json:"timeline_exclude"`
 	CreatedAt   string         `json:"created_at"`
 	UpdatedAt   string         `json:"updated_at"`
 	Links       []linkView     `json:"links,omitempty"`
@@ -429,6 +432,7 @@ func toItemView(it *store.Item) itemView {
 		PublicLiveURL:     it.PublicLiveURL,
 		PublicSourceURL:   it.PublicSourceURL,
 		PublicScreenshots: sliceOr(it.PublicScreenshots, []string{}),
+		TimelineExclude:   sliceOr(it.TimelineExclude, []string{}),
 		CreatedAt:         it.CreatedAt.UTC().Format("2006-01-02T15:04:05Z"),
 		UpdatedAt:         it.UpdatedAt.UTC().Format("2006-01-02T15:04:05Z"),
 	}
@@ -668,6 +672,7 @@ func updateItemTool(st *store.Store) ToolHandler {
 		PublicLiveURL     *string   `json:"public_live_url"`
 		PublicSourceURL   *string   `json:"public_source_url"`
 		PublicScreenshots *[]string `json:"public_screenshots"`
+		TimelineExclude   *[]string `json:"timeline_exclude"`
 	}
 	return func(ctx context.Context, raw json.RawMessage) (any, error) {
 		var in input
@@ -693,6 +698,7 @@ func updateItemTool(st *store.Store) ToolHandler {
 			PublicLiveURL:     it.PublicLiveURL,
 			PublicSourceURL:   it.PublicSourceURL,
 			PublicScreenshots: it.PublicScreenshots,
+			TimelineExclude:   it.TimelineExclude,
 		}
 		if in.Title != nil {
 			patch.Title = *in.Title
@@ -733,6 +739,24 @@ func updateItemTool(st *store.Store) ToolHandler {
 		if in.PublicScreenshots != nil {
 			patch.PublicScreenshots = *in.PublicScreenshots
 		}
+		if in.TimelineExclude != nil {
+			// Whitelist values so a stray entry doesn't poison the array. Same
+			// allowlist as parseTimelineExcludeList in web/.
+			allowed := map[string]struct{}{"todos": {}, "events": {}, "docs": {}, "creation": {}}
+			out := make([]string, 0, len(*in.TimelineExclude))
+			seen := map[string]struct{}{}
+			for _, v := range *in.TimelineExclude {
+				if _, ok := allowed[v]; !ok {
+					continue
+				}
+				if _, dup := seen[v]; dup {
+					continue
+				}
+				seen[v] = struct{}{}
+				out = append(out, v)
+			}
+			patch.TimelineExclude = out
+		}
 		if in.ParentPaths != nil {
 			pids, err := resolveParentPaths(ctx, st, *in.ParentPaths)
 			if err != nil {