Generate CLI reference docs

[nathanjcochran]

This adds generated Markdown reference documentation for every CLI command, following the same approach ghost already uses. A small cmd/generate-docs tool walks the Cobra command tree and writes one page per command to docs/cli/, so the reference always reflects the real help text rather than something maintained by hand.

Generation is wired into go generate ./..., alongside the existing API client and mock generation. That gives contributors a single command to remember, and it means the existing CI check now fails whenever a command, flag, or help text changes without the docs being regenerated. The CI step also picks up newly generated files, so a new command can't land without its reference page.

To keep the output identical across machines, the default config directory is now shown in help as ~/.config/tiger and expanded when the config loads, instead of being expanded up front into a machine-specific path.