The @ popover only completed filesystem references; bot handles worked
when fully typed (mention middleware parses at submit) but were never
offered, so users had to know the exact handle — worse with multi-source
@name-device handles. Fixes#88060 (ported from Hermes-Bot-Mode#43).
- composer contrib: new 'composer.atCompletions' data area
(ComposerAtCompletionSource) — contributed rows merge AHEAD of path
results; a throwing source drops its rows, never the popover
- use-at-completions: merge contributed entries in all three fetch paths
(gateway results, gateway-less, fetch error)
- SDK: export the new area + types for plugins
- bundled Bot Mode plugin: registers 'mention-completions' — roster
handles from the query cache (\u22645s stale), active profile excluded,
'default' offered as @hermes, multi-source @name-device handles via
botHandle, display name + connection label in the row meta, capped at 8
- registered early in register(ctx) so vm harnesses reach it before the
pane/UI registrations that stubs can't fully model
1a74e7fb43