refactor(cli): promote --demo flag to demo subcommand

[cpcloud]

Summary

• Promote --demo from a root-level flag to a demo subcommand
• Move --years to the demo subcommand (no longer requires a separate --demo prerequisite)
• Split TUI launch into runTUI (normal) and launchTUI (shared core) with runDemo handling demo-specific :memory: default
• Update all docs, blog posts, VHS tapes, issue templates, and plans to use micasa demo syntax

Before: micasa --demo --years 10 /tmp/perf.db
After: micasa demo --years 10 /tmp/perf.db

[codecov]

Codecov Report

❌ Patch coverage is 55.55556% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.53%. Comparing base (e085133) to head (5662362).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/micasa/main.go	55.55%	15 Missing and 1 partial ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
cmd/micasa/main.go	48.74% <55.55%> (+1.09%)	⬆️

... and 4 files with indirect coverage changes

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