Moves buildAgentMenu's DOM-building body, the menuItem/menuSep/menuLink
helpers, agentMenuPost, and the open-dropdown coordination logic out of
swarm.js and into a new <hive-agent-menu> autonomous custom element
(dashboard/src/agent-menu/), following the same shadow-DOM + one-dir-per-
component shape as hive-dialog. swarm.js's buildAgentMenu is now a thin
wrapper that constructs the element and sets ._opts before appending it,
same convention hive-dialog uses since a custom element created via
document.createElement can't take constructor args.
The module-level "one dropdown open at a time" singleton (previously a
single mutable variable in swarm.js) becomes a tracked Set of open
instances inside the component module; each instance closes itself via
its own close() method rather than another instance reaching into its
shadow internals. The document-level outside-click and Escape listeners
move into the component module too, keyed off e.composedPath() instead of
e.target.closest() -- shadow-DOM event retargeting means a plain
e.target check no longer reliably reaches into a specific instance's
shadow tree. closeAllAgentMenus() is exported for swarm.js's
buildAgentTree, which still needs to close any open menu before it
replaces the container tree DOM.
The hover-reveal opacity rule crosses the shadow boundary via a
--menu-btn-opacity custom property (custom properties inherit through
shadow boundaries): dashboard.css sets it on hover of the light-DOM
hive-agent-menu element, and the component sets it directly from JS while
its own dropdown is open, since that's component-internal state a CSS
selector out in the light DOM can't see. The host element itself takes on
the structural role (flex:none, position:relative, ...) the old
light-DOM .agent-menu wrapper div played, since its shadow tree's
absolute-positioned dropdown needs a positioned ancestor to anchor off of.
Verified end to end with a standalone esbuild-bundled test harness run
under headless chromium: row layout/flex sizing, hover-reveal opacity,
and dropdown positioning all render correctly, and a scripted interaction
pass (singleton exclusivity, outside-click close, Escape close, toggle
behavior, menu-item click close, and the exported closeAllAgentMenus())
all pass.
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