feat: add yc command completions

[oz-for-oss]

Closes #5781

Summary

• Add a local yc command signature to Warp's legacy completion registry.
• Use yc __completeNoDesc to ask the installed Yandex Cloud CLI for nested completions.
• Provide static root command groups and common global flags for initial suggestions.
• Cover the new behavior with focused warp_completer tests.

Validation

• ./script/format
• CARGO_HOME=/workspace/.cargo cargo test -p warp_completer --features test-util test_yc -- --nocapture
• CARGO_HOME=/workspace/.cargo cargo clippy -p warp_completer --all-targets --all-features --tests -- -D warnings
• Attempted CARGO_HOME=/workspace/.cargo cargo test -p warp_completer --features test-util -- --nocapture; unrelated existing tests fail while checking FeatureFlag::CloudEnvironments before feature flags are initialized.
• Attempted CARGO_HOME=/workspace/.cargo cargo clippy --workspace --all-targets --all-features --tests -- -D warnings; workspace clippy fails in unrelated app/src/ai/execution_profiles/profiles_tests.rs all-features test imports/methods and existing dead-code warnings before completion.

Notes

• Workspace-wide clippy currently stops on unrelated all-features app test issues before finishing; the changed warp_completer crate passes clippy.

[warp-agent-staging]

Closing as superseded.

yc completions shipped instead in warpdotdev/command-signatures#303 (merged). Rather than a hand-written static signature in crates/warp_completer, the spec lives in the command-signatures repo as a generator that shells out to the CLI's own yc __complete, matching how other Cobra CLIs (kubectl, oc) are handled. Completions therefore track the installed yc binary and carry its descriptions, rather than being frozen in the client.

Thanks — the investigation here (particularly identifying yc's Cobra completion endpoint) fed directly into that implementation.