fix(cli): emit json for missing agent show

[OnlyYu1996]

Fixes cortex agent show <missing> --json so the command emits a JSON error object on stdout instead of only relying on the plain text top-level error.

Bounty issue: PlatformNetwork/bounty-challenge#53103

Changes:

• Add shared missing-agent error helpers for agent show.
• When --json is set, print { "error": "Agent '<name>' not found" } before returning the existing non-zero error.
• Keep non-JSON 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::handlers::show::tests::test_agent_not_found_json_error -- --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 agent show nonexistent --json stdout JSON parsing and non-JSON behavior
• git diff --check

[greptile-apps]

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