Folds the backend half of #113396 (#89720) into this branch so there is ONE
friendly-name resolver: `_display_name` reads the Bot Mode title, then the
profile.yaml `display_name` written by `hermes profile rename` (the Desktop's
botFriendlyNames order), then the @handle — so the renamed primary signs
`Maia (@hermes)` instead of `hermes (@hermes)`. Reaching it by `maia` /
`@maia` already works through `_resolve_local_name`, which stays the single
resolver (fail-closed on ambiguity, punctuated names accepted).
`local_alias_map` docstring now states what the code does: an exact folder
id wins by design; a friendly name colliding with another folder id never
steals it, and ambiguity is only between friendly names.
Docs: the Direct-messages bullet lists the accepted target forms (profile
name, friendly name, Desktop @-slug) and the fail-closed ambiguity rule.
Review follow-up for #113410 / #113396.
58c6f6fa20