fix: include "Finished" line in cargo build/check success output

[mvanhorn]

Summary

Preserves the cargo Finished ... line in the filtered success output for rtk cargo build and rtk cargo check. Previously, success output only showed cargo build (0 crates compiled) which was ambiguous - LLMs couldn't tell if the build succeeded or if there were errors.

Why this matters

Per #759, Claude interpreted cargo build (0 crates compiled) as potentially erroneous and tried workarounds (rtk proxy cargo check, RUSTFLAGS="-D warnings" rtk proxy cargo check), consuming extra tokens. Claude expected to see the Finished dev profile ... message as a clear success signal.

Changes

• src/cargo_cmd.rs: In filter_cargo_build(), capture the Finished line instead of skipping it, and append it to the success summary. The output now looks like:

  cargo build (5 crates compiled)
  Finished dev [unoptimized + debuginfo] target(s) in 2.53s
  

Testing

All 38 cargo_cmd tests pass (cargo test cargo_cmd). The change only affects the success path - error/warning output is unchanged.

Fixes #759

This contribution was developed with AI assistance (Claude Code).

[CLAassistant]

All committers have signed the CLA.

[aeppling]

Hello, thanks for contributing with this fix,

Please target develop branch and read CONTRIBUTING.md so you can pass all checks,

[mvanhorn]

Rebased onto develop and retargeted the PR base. Thanks for the heads up on CONTRIBUTING.md.