feat: Migrate to the new update-flag contract, and flag update --weight (#92)

Author: khvn26

README.md

@@ -56,10 +56,12 @@ flagsmith flag list    # list the flags in the current environment
 
 ## Commands
 
+Reading works against any Flagsmith instance. Changing flags — `flag update`, `flag enable`/`disable`, `flag reorder`, `flag delete` — needs Flagsmith 2.263.0 or newer, self-hosted or SaaS.
+
 - `flagsmith init` — bind the current directory to a project (writes `flagsmith.json`).
 - `flagsmith flag list` — list feature flags in the current environment.
 - `flagsmith flag get <feature>` — show a single flag's state (`--segment <id>` or `--identifier <id>` for an override).
-- `flagsmith flag update <feature>` — toggle (`--enable`/`--disable`) or set the value (`--value`, `--type`); `--segment <id>` or `--identifier <id>` targets an override.
+- `flagsmith flag update <feature>` — toggle (`--enable`/`--disable`), set the value (`--value`, `--type`), or re-weight a multivariate flag's variants (`--weight <key|id>=<percentage>`); `--segment <id>` or `--identifier <id>` targets an override.
 - `flagsmith flag enable|disable <feature>` — shorthand for `flag update --enable`/`--disable` (same `--segment`/`--identifier` targeting).
 - `flagsmith flag delete <feature> --segment <id>|--identifier <id>` — delete a segment or identity override.
 - `flagsmith segment list` — list segments (`--include-feature-specific` to include feature-scoped ones).