fix(cli): add dry run for agent copy

[OnlyYu1996]

Adds a preview-only path for cortex agent copy.

Bounty issue: PlatformNetwork/bounty-challenge#53089

Changes:

• Add --dry-run to CopyArgs.
• Let dry-run pass destination-exists checks and report whether --force would be required.
• Print resolved source/destination and existence/force status without creating the agents directory or writing the destination file.
• Keep normal copy behavior unchanged.
• Fix the current main read_file_with_encoding test import so agent command tests compile.

Validation:

• cargo +stable fmt --package cortex-cli --check
• cargo +stable test --package cortex-cli agent_cmd::tests::tests::test_copy_args_dry_run -- --nocapture
• cargo +stable test --package cortex-cli agent_cmd::tests -- --nocapture
• cargo +stable check --package cortex-cli --lib
• cargo +stable build --package cortex-cli --bin Cortex
• real CLI smoke for help text, dry-run output, no destination file write, and existing-destination Force required: yes
• git diff --check

[greptile-apps]

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