Fix ssh_exec error reporting + circular import in switch_profile ...

ssh_exec:
- Non-zero exit status: output (stdout/stderr) now included in error
  field so model can actually see why the command failed
- All error cases: error field now contains human-readable message
  instead of short code (no_target, permission_denied, timeout)
- Named connections from ssh_hosts.json: default known_hosts=none
  same as ad-hoc connections (was missing, caused host key failures)

switch_profile:
- Move ProfileSwitched import inside execute() to break circular
  import: tools.__init__ → switch_profile → core.events → core.agent
  → core.registry → tools

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Profile switch: emit WS event so client updates UI immediately ...

ProfileSwitched event emitted by switch_profile tool via current_event_sink.
Client handles profile_switched: updates chat header, profile selector,
and local sessions[] — no page refresh needed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Add switch_profile tool for automatic profile switching ...

Navi can now switch her own profile mid-session when the task domain
changes. The new profile (tools + system prompt) takes effect from the
next user message. Injected with session_store + profile_registry like
SpawnAgentTool. Added to all profiles' enabled_tools and persona.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>