Symptom: a model-provider plugin installed with `hermes plugins install`
(e.g. claude-subscription-directsdk) worked from a terminal but the Desktop
app failed every session build with "Unknown provider '<name>'".
Cause: `providers/__init__.py` scanned `$HERMES_HOME/plugins` exactly once
per process, under whichever profile home happened to be bound at the first
lookup, and the registry was global. The Desktop backend and the multiplex
gateway serve several profiles from one process, so any profile other than
the first-discovered one never saw its own plugins, and a plugin installed
while the process ran was invisible until a restart.
Change: bundled, pip and legacy providers stay process-wide; `$HERMES_HOME`
plugins load into a per-home layer keyed by `hermes_home_key()` at lookup
time (`get_provider_profile`, `list_providers`, `provider_source`). The
layer rescans when the plugin directories' mtimes change, so a fresh install
is found on the next lookup. The registration target is a ContextVar so two
turn threads scanning two homes cannot cross-register, and no lock is held
across plugin imports (a lock there could deadlock against a thread
mid-`import hermes_cli.auth`). Per-home module names let two profiles carry
the same plugin. `plugin_dev` reuses the module-name helper.
Live repro (tui_gateway `session.create` on a secondary profile whose config
selects a plugin installed only there): base -> agent_error "Unknown
provider 'fakeprov-b'"; fixed -> provider resolves and the build proceeds to
the plugin's own runtime check.
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