docs: document Prop 121 disabling outbound IBC, remove dead bridge-out routes and IBC remnants - #51

Accurate, well-scoped docs update that replaces the now-impossible "bridge out before the vote" guidance with the post-Prop-121 reality across all 13 surfaces, including the llms.txt generator. No blockers; a likely-broken in-page anchor, one stale future-tense date, and stale committed llms.txt artifacts are worth fixing.