Closes the last options-field rehash deferral (carried since slice 91): an operator edits options { default-channel-modes } on disk, REHASHes, and a channel created after the rehash is stamped with the edited default — no restart. Existing channels keep their creation-time modes (the default is a creation-time stamp, not a live channel property; the oracle's rehash() does not retroactively re-mode either). Threads the value through the existing ConfStore via the slice-91 live-or-ctx idiom so both runtime read sites (Channels::join's internal stamp and command/join.rs's authoritative peer MODE relay) prefer the live value and fall back to their boot value when the global was never seeded (unit tests). No change to rehash.rs's control flow — rehash_config() already re-seeds. Tests: control units (config_conf extract + seed/getter/reseed/clear), rehash_conf_proptest extended to track default_channel_modes in lockstep, and golden_rehash_channel_modes boot test driving the live global path end-to-end (incl. the no-retroactive-re-mode + revert inverses).