Deprecate shell command and hide from help output (#243)

## Summary

Deprecates the `shell` command (`mcpc shell @<session>` and `mcpc
@<session> shell`) by hiding it from help output and adding a
deprecation warning when invoked. The command will be removed in a
future release.

## Changes

- **Hidden from help**: Both `shell` command variants are now marked as
hidden in the CLI command definitions, removing them from `--help`
output
- **Deprecation warning**: Added a deprecation warning message that
prints to stderr when the `shell` command is invoked
- **Updated documentation**: 
  - Added deprecation notice to CHANGELOG.md
- Updated README.md help output to reflect the current command set
(without `shell`)
- Removed `shell` from the "Available commands" section in server
details output
- **Test updates**: Updated unit tests to reflect that `shell` is no
longer listed in available commands and is not shown when a server has
no capabilities

## Implementation Details

- The deprecation warning uses the existing `formatWarning()` utility
for consistent styling
- The "Available commands" section in server details is now only shown
if there are actual commands to display (previously always shown even if
only `shell` was available)
- Both top-level (`mcpc shell @<session>`) and session-level (`mcpc
@<session> shell`) command variants are deprecated consistently

https://claude.ai/code/session_01UvWPJHftasQatWBX7Uisbh

Co-authored-by: Claude <noreply@anthropic.com>