Issue identifiers (`MUL-123`) are recomputed from `workspace.issue_prefix`
at read time, so cached issues kept showing the old `OLD-N` keys after a
prefix change. Without invalidation the confirm dialog's "all issues will
be renumbered" promise was broken until a hard refresh — and other tabs
receiving the `workspace:updated` WS event saw the same drift.
- WorkspaceTab: after a prefix-changing save, invalidate `issueKeys.all`
in addition to the workspace list. Non-prefix saves stay cheap.
- Realtime: split `workspace:updated` out of the generic `workspace`
refresh into a specific handler that compares cached vs incoming
`issue_prefix` and invalidates issues only when it actually changed.
- Docs: align the "uppercase" language with the actual UI/backend rule
(uppercase letters and digits, up to 10 chars).
Co-authored-by: multica-agent <github@multica.ai>
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