fix(refacto-p1): unified cmds execution flow (+ rm dead code)

[aeppling]

Part 2 of codebase refacto

Summary

• Standardize exit code handling across all modules: Result<()> → Result with a single process::exit() in main.rs — modules
  never call process::exit() directly
• Standardize filter execution flow across commands
• Fix critical bug in runner.rs where rtk err and rtk test always exited 0 regardless of command failure
• Add exit_code_from_status() helper for passthrough commands and centralize signal handling (128+signal) in
  exit_code_from_output()

Test plan

• cargo fmt --all && cargo clippy --all-targets && cargo test — 1123 passed, 0 failed
• Zero std::process::exit() in src/cmds/ and src/core/ (verified via grep), exactly 1 in main.rs
• golangci-lint exit 1 exception preserved (lint issues → exit 0, matching old behavior)
• src/cmds/README.md updated with new Result contract, examples, and exit code tables
• Manual testing: rtk ; echo $? for success (0) and failure (non-zero) paths