`/skills browse` is documented to scan 6 sources and take ~15s, but the
gateway dispatched `skills.manage` on the main RPC thread. While it
ran, every other inbound RPC — completions, new slash commands, even
`approval.respond` — blocked until the HTTP fetches finished, making
the whole TUI feel frozen. Reported during TUI v2 retest:
"/skills browse blocks everything else".
`_LONG_HANDLERS` already exists precisely for this pattern (slash.exec,
shell.exec, session.resume, etc. run on `_pool`). Add `skills.manage`
to that set so browse/search/install run off the dispatcher; the fast
`list` / `inspect` actions pay a negligible thread-pool hop.
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