feat(cli): add startup model option

[birdmanmandbir]

Summary

• add --model MODEL and TACT_MODEL to select the model for a newly started agent
• apply the selected model to a fresh interactive session and to tact run
• keep the default model unchanged when no override is provided
• document the option and cover CLI parsing and global-option behavior

Scope

• Keep model selection per launch; do not persist it to Tact configuration.
• Preserve the model recorded by an existing session when using --resume or selecting a saved session.
• Reuse Nanocodex's existing model parser and supported model set; do not add a Tact-specific model registry.

Validation

• cargo test -p tact app::cli::tests
• cargo check --all-features
• cargo fmt --all -- --check
• cargo run -p tact -- --help

[codspeed-hq]

Merging this PR will not alter performance

✅ 18 untouched benchmarks
⏩ 31 skipped benchmarks¹

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_{Comparing birdmanmandbir:feat/startup-model (6813430) with main (a2de8ae)}

Footnotes

1. 31 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩