feat: add --agent flag to print detected package manager name

[Luc0-0]

Closes #326

Problem

There's no way to get just the detected package manager name from ni without parsing the full ni -v table output. This makes it awkward to use in CI pipelines and shell scripts where you need the bare agent name for conditional logic.

Solution

Adds a --agent flag that prints only the detected package manager name to stdout — no colors, no version info, just the agent name followed by a newline.

$ ni --agent
pnpm

$ ni --agent
bun

Works with any ni command since it's handled in the runner before agent-specific logic runs. Prints unknown if no lock file is found and no default is configured.

Also updated the help output (ni -h) to document the new flag.

Use case

# capture in a variable
PM=$(ni --agent)

# use in CI conditionals
if [ "$(ni --agent)" = "pnpm" ]; then
  pnpm install --frozen-lockfile
fi