Rebrand the daemon and workspace: the binary is now `echo`, every crate
is `echo-*` (`echo-api` and the module crates), Rust imports use
`echo_*`, the gRPC proto is `proto/echo.proto` with package `echo.v1`,
and the log filter, gossip peer-name default, and on-disk store default
(`echo.db.jsonl`) follow. Docs updated throughout. No behaviour change;
crate directories and the config schema are untouched.
POST/DEL/LIST each move to their own file (each still parameterised by
bulletin kind, so OPOST/ODEL/OLIST route to the same handler); the kind
constants stay in lib.rs beside the dispatcher. No behaviour change.
