Move CLI commands into testable functions and test them (closes #12)

The command bodies in bin/quak.ts become functions in
src/cli-commands.ts that take their options and a context (output
streams, session directory, cache directory, session loader) and return
an exit code. bin/quak.ts wires them to commander and exits with that
code once stdout and stderr have drained; nothing below it calls
process.exit. test/cli/commands.test.ts drives the commands with a fake
client and temp directories. Output is unchanged.

Model: opus-5-5

TODO.md

@@ -18,6 +18,15 @@ Tag v1.0.0.
 
 # Completed Steps
 
+- 2026-09-23: Made the CLI testable and tested it (issue 12). The command bodies
+  moved from `bin/quak.ts` into `src/cli-commands.ts` as functions that take
+  their options and a context (output streams, session directory, cache
+  directory, session loader) and return an exit code; `bin/quak.ts` only wires
+  them to commander and exits with the code once stdout and stderr have drained,
+  so nothing below it calls `process.exit`. `test/cli/commands.test.ts` drives
+  them with a fake client: session file modes, logout, the missing and corrupt
+  session paths, and the output and exit code of `whoami`, `collections`,
+  `files`, `get`, `get-thumb`, `backup` and `helper list-missing-thumbnails`.
 - 2026-09-23: Hardened the backup tree's atomic copy (issue 22). `copyAtomic`
   fsyncs its temp file before the rename and the directory after it, through the
   download writer's `fsyncPath`; each backup run deletes `.quak-backup-*.tmp`