fix: add -r short alias for slug option in pr-base-picking

[sentry]

The pr-base-picking command defined --slug without the -r short alias, unlike the global options shared across other CLI commands (commit, upload, etc.). This caused invocations specifying -r <slug> (such as verification scripts and standard command usage) to fail with Error: No such option '-r'.

This change:

• Adds -r as an alias to --slug on pr_base_picking in codecov_cli/commands/base_picking.py.
• Updates the generated codecovcli_commands reference text.

Fixes CLI-J0

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[codecov]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
723	1	722	1

View the full list of 1 ❄️ flaky test(s)

api.temp.calculator.test_calculator::test_divide

Flake rate in main: 100.00% (Passed 0 times, Failed 1726 times)

Stack Traces | 0.001s run time

def
                test_divide():
                > assert Calculator.divide(1, 2) == 0.5
                E assert 1.0 == 0.5
                E + where 1.0 = <function Calculator.divide at 0x104c9eb90>(1, 2)
                E + where <function Calculator.divide at 0x104c9eb90> = Calculator.divide
                .../temp/calculator/test_calculator.py:30: AssertionError

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