Add flag syntax to help prompt and improve help CLI UX
Add documented flag syntax (--day, --force, --reprocess, etc.) to
muse/help.md command reference so the help agent stops hallucinating
positional args. Add guardrail instruction. Add more example Q&As.
Add stdin mode to sol help: when invoked with no args, reads a question
from stdin (with interactive prompt on terminals, silent for pipes).
Add "Thinking..." progress indicator to stderr during agent processing,
using streaming Popen instead of blocking subprocess.run.