docs: scrub selection-bar #443/#596 cookies in tabs.js, add interaction-model preface (#712 batch 3)

selection + selection-bar in tabs.js carried six #443 cookies + two
#596 cookies — most marking the bulk-bar architectural decision
('actions live in bar, not on per-card buttons; manager not
special-cased') or the SW4RM-tab gate. All scrubbed; the canonical
docs/web-ui.md::Selection bar section already covered the bulk
actions in detail but was missing a preface for the **interaction
model** itself (how selection enters/exits, why per-card buttons
moved). Added that as the opening paragraph of the section so the
in-code refs have something to point at.

Net in this batch:
- tabs.js: -8 #NNN refs (#443 ×6, #596 ×2)
- docs/web-ui.md: +9 lines (interaction-model preface)
- functional code unchanged; build clean

refs #712

docs/web-ui.md

@@ -504,6 +504,15 @@ stops at the row's icon midline).
 
 ### Selection bar
 
+Per-card action buttons (`R3ST4RT` / `ST0P` / `ST4RT` / `R3BU1LD` /
+`DESTR0Y` / `PURG3`) used to live on each container row; the
+operator picked the bulk-bar model instead. Clicking an agent's
+icon toggles its selection (an in-memory `Set<name>`); `Esc` or
+the bar's `✕ clear` button drops everything. The selection
+persists across tab switches in-memory — the bar just hides on
+non-SW4RM tabs since other tabs don't show the agent cards needed
+to cross-reference.
+
 When one or more agents are selected (via icon click), a sticky
 frosted-mauve bar slides up from the bottom of the viewport
 (`#selection-bar`, `position: fixed; bottom: 0`). It shows: