Refresh the README: the quorum it describes is a join plugin now, and versioning, retention and four plugin types are missing

The README ships inside every tag and is the first thing a reader sees on the project page and in the repository, and nothing reads it back: no test, no linter and no CI job compares it to the module. It now describes routing the engine no longer does that way, and leaves out most of what the module has grown since.

The routing bullet still says a quorum ("two of three approved") is a wait_all join, a merge and a count condition, with no special node. That stopped being true when the advanced joins shipped: ThresholdJoin, QuorumJoin, TimeoutJoin and MatchingJoin are plugins, and a quorum is one of them rather than three pieces assembled by hand. The bullet now names the joins that ship.

The plugin bullet lists five plugin types. Four more are attribute-based plugin types a site extends the same way: NodeFeature (what a node carries), VariableProvider (the values a run computes), DeadlineProvider (when a wait runs out) and TimeoutAction (what happens once it has).

The feature list has five entries and misses six capabilities that each have a handbook chapter: content-bound and automated work, versioning, timeouts and incidents, payment, operations and audit, and retention. Versioning and retention are the two worth calling out, because a reader deciding whether to adopt Orchestra cannot tell from the README that an instance runs against an immutable snapshot of the workflow it started on, or that finished instances are purged on a configurable age. Each new bullet is worded from the chapter it summarizes.

The orchestra_domain row describes tenant resolution only. Since #3613775: Task notifications link to whichever site caused the task, not to the one people work on the submodule also keeps every link people follow on the domain it belongs to: a notification links to the domain its process started on, an inbox link to the domain people work on.

README only, no code change.

AI-Generated: Yes (Claude Code compared the README against the module's plugin types, join plugins, submodule descriptions and handbook chapters, and drafted the replacement prose and this summary. I reviewed both.)