feat: add Astra model support

[clabby]

Summary

• update Nanocodex to commit 4f3344400ee3d2a2446b96e342f8c9871aab9637
• adapt Tact to the upstream API changes
• add Astra to the model selector and model-specific presentation
• support astra through --model, TACT_MODEL, and [agent].model

Dependency

This PR is blocked on a new Nanocodex release containing 4f3344400ee3d2a2446b96e342f8c9871aab9637. The dependency is pinned directly to that Git commit until the release is available; update it to the released crate version before marking this PR ready.

Testing

• just check-fmt
• cargo check --all-features
• just clippy
• just test — 962 tests passed

[codspeed-hq]

Merging this PR will not alter performance

✅ 18 untouched benchmarks
⏩ 31 skipped benchmarks¹

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_{Comparing cl/astra (6863e6f) with main (767f81a)}

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. ↩