Adds PlatformConfig.typing_status_text for the two platforms that render
text for the working-state line: Slack's assistant.threads.setStatus
status (hardcoded 'is thinking...') and Google Chat's visible marker
message (hardcoded 'Hermes is thinking…'). None keeps each platform's
built-in default; to_dict omits the field when unset so existing configs
serialize unchanged. Plumbing mirrors typing_indicator exactly (typed
field, from_dict extra fallback, shared-key bridge).
Also documents that Slack's status line requires the assistant:write
scope — without it setStatus fails silently and Slack shows its own
generic placeholder, which previously made the behaviour undiagnosable
from config alone.
dc0c778b22