Move the hashcash proof-of-work stamp from the X-Hashcash HTTP header
into the JSON request body as a 'hashcash' field on POST /api/v1/session.
Updated server handler, CLI client, SPA client, and documentation.
Move all non-bootstrapping CLI code to internal/cli package.
cmd/neoirc-cli/main.go now contains only minimal bootstrapping
that calls cli.Run(). The App struct, UI, command handlers, poll
loop, and api client are now in internal/cli/ and internal/cli/api/.
