Route all CLI output through the command's stdout/stderr

[nathanjcochran]

Adopts Ghost's output conventions. Commands now print with cobra's cmd.Print* / cmd.PrintErr* helpers instead of fmt.Print* or spelled-out fmt.Fprintf(cmd.OutOrStdout(), …), and the root command wires its writers to stdout/stderr explicitly — cobra's Print* silently falls back to stderr otherwise (which is why tiger upgrade was sending all its progress to the wrong stream). Helpers inside internal/cmd take the command and print through it; code in internal/common and internal/version still takes an io.Writer so it stays free of cobra.

The payoff is that every byte a command emits now flows through writers we control, so stream choice is auditable and all output is capturable in tests (I intend to refactor our testing patterns in a later PR, and this helps set the stage for it). Additionally, using the cmd.Print* helpers is just cleaner, less verbose, and easier to read overall (imo). Finally, the cmd.Print* helpers do not return an error (unlike fmt.Fprint*), which is helpful in case we ever want to add errcheck to the CI pipeline (but we're still a long way from that, currently 😅).

This PR also codifies the rule for what goes where: stdout carries the actual command result/payload, whereas stderr carries warnings, progress updates, and anything interactive. Anything that previously did not adhere to that rule has been updated.

Bug fixes and behavior changes that came out of it:

• tiger mcp install's success banner and tiger auth login's credential prompts were writing straight to the process's stdout, bypassing the command entirely and therefore invisible to tests.
• tiger db test-connection printed its own diagnostic and then let cobra print the raw error a second time, plus the full usage block. It now reports once.
• tiger upgrade's progress output moves from stderr to stdout.
• Interactive prompts in auth login and db save-password move to stderr, so they stay visible when stdout is redirected.
• The whole auth login OAuth flow — auth URL, browser warnings, and the project picker — moves to stderr. Redirecting stdout previously hid the picker entirely while it sat waiting for keystrokes.

The conventions are written up in CLAUDE.md.

[Askir]

I think CI/CD is red due to a flake. Looks good to me though!