fix: make export pretty flag explicit

[SlaVKsVolks]

Fixes PlatformNetwork/bounty-challenge#53204.

Changes:

• Makes export --pretty an explicit opt-in instead of defaulting JSON output to pretty formatting.
• Narrows the help text to JSON output only.
• Rejects --pretty for YAML/CSV so the flag is no longer silently ignored for those formats.
• Adds parser/validation coverage for the new behavior.

Verification:

• rustfmt --edition 2024 --check src/cortex-cli/src/export_cmd.rs
• cargo check -p cortex-cli
• git diff --check

Note: targeted cargo test for export_cmd currently fails before reaching these tests because src/cortex-cli/src/agent_cmd/tests.rs imports private read_file_with_encoding on origin/main.

[greptile-apps]

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