Adds `messages.responseUsage` (precedence session -> channel -> config default
-> off) so the persistent /usage footer can default-on, with three distinct
states: explicit on (tokens/full), explicit off (persisted), and unset (inherit
the configured default).
Unifies effective-value resolution behind a single channel-aware resolver
`resolveEffectiveResponseUsage` used by reply rendering, the no-arg /usage
toggle, the ACP control, and the gateway session-row builder; the row builder's
`effectiveResponseUsage` is carried through sessions.changed events, chat
snapshots, and the UI row so live consumers never go stale. `/usage reset`
(aliases inherit/clear/default) clears the override to inherit; only explicit
off persists; a full session reset preserves the preference. ACP "Usage detail"
gains an "inherit" option for unset sessions. Docs/help/completions updated; "on"
documented as a legacy alias; config-doc baseline regenerated.
6add1cc969