`hermes update` folds an eligible multi-profile install onto one multiplexed
gateway on its own, and there is currently no way to say no. The only lever,
`gateway.multiplex_profiles: false`, is also the default: `_read_multiplex_flag`
returns `False` for "absent" and for an explicit `false` alike, so an operator
who has already decided to stay on per-profile gateways has no way to record
that decision. The migration runs again on the next update.
Add `gateway.auto_migrate` (bool, default `true`). Read from the default
profile's config, it gates the automatic path only:
- absent or `true` -> today's behaviour exactly, no change
- `false` -> `maybe_auto_migrate_after_update()` returns before
building a plan; no output, no changes
`hermes gateway migrate --multiplex` is an explicit request and still migrates
regardless of the flag, so it stays the supported way to opt back in.
One early return, one schema entry with the reasoning inline, one invariant
test (opt-out blocks the hook, absent/true do not, explicit command still
applies), one section in the multi-profile gateways guide.
Athena
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