Import YAML session exports

[GreyforgeLabs]

Summary

• Allow cortex import to parse session exports as JSON or YAML.
• Preserve helpful parse diagnostics for invalid content while updating them to mention JSON/YAML exports.
• Add a binary integration regression that imports a YAML session export produced in the same shape as cortex export --format yaml.

Fixes PlatformNetwork/bounty-challenge#44938.

Validation

• cargo test -p cortex-cli --test import_yaml --no-default-features --features cortex-tui -- --nocapture
• cargo check -p cortex-cli --no-default-features --features cortex-tui
• cargo fmt --package cortex-cli --check
• git diff --check

[greptile-apps]

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