Add cp command to copy files out of containers, binary-safe

`portainer-cli cp <stack> <container>:<path> <dest>` goes through Docker's
archive API, so unlike `console --run "cat …"` there's no TTY in between and
binary files (database dumps) arrive intact. Works like `docker cp` for files
and directories; a dest of "-" writes a single file to stdout.

Stack and container lookup moves from Console into a shared Resolver module.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -14,6 +14,7 @@ A Ruby command-line tool for interacting with [Portainer](https://www.portainer.
 - Supports both API key and JWT token authentication
 - Configurable SSL verification (useful for self-signed certificates)
 - `--run CMD` flag for running a specific command (e.g. `rails c`)
+- `cp` to copy files and directories out of containers, binary-safe
 - `--quiet` flag for scripting and AI agent use cases
 
 ## Requirements
@@ -98,6 +99,16 @@ Suppresses all status output — only the container's own stdout/stderr is print
 portainer-cli console myapp --quiet --run "rails runner 'puts User.count'"
 ```
 
+### Copy files out of a container
+
+Like `docker cp`, through Docker's archive API. There's no TTY in between, so binary files (database dumps, images) arrive intact — which `console --run "cat …"` can't promise.
+
+```sh
+portainer-cli cp myapp backup:/backups/latest.dump ./latest.dump
+portainer-cli cp myapp app:/app/log ./                 # directory → ./log
+portainer-cli cp myapp app:/etc/hostname - --quiet     # single file to stdout
+```
+
 ### Create resources
 
 ```sh
@@ -124,7 +135,7 @@ portainer-cli exec <endpoint-id> <container-name-or-id> --shell /bin/bash
 
 ## Container name resolution
 
-When using `console`, names are resolved in this order:
+When using `console` or `cp`, names are resolved in this order:
 
 1. **Exact match** — full container name or ID prefix
 2. **Compose pattern** — `<stack>-<service>-<N>` (e.g. `librarian` → `librarian-librarian-1`)