feat: add yarn workspace command with smart filter routing

[denneulin]

Summary

Add rtk yarn workspace <pkg> [run] <script> command that strips yarn boilerplate and routes script output
through existing specialized filters for maximum token savings.

• Boilerplate stripping: Removes YN-prefixed info lines, resolution/fetch/link progress, yarn classic
  headers (yarn run v1.x, Done in Xs, info lines). Returns ok ✓ when output is pure boilerplate.
• Smart filter routing: Routes workspace scripts to existing RTK filters based on script name:
  • vitest → vitest parser, tsc/typecheck → tsc filter, lint/lint:* → generic lint filter,
    prettier/format → prettier filter, test/test:* → test runner summary
  • Prefix exclusions prevent misrouting (test:e2e, test:playwright, test:cypress, lint:fix →
    passthrough)
• Graceful degradation: catch_unwind on filter panics, empty-output guard, fallback to stripped output
  with stderr warning
• Early-exit on failure: When yarn itself fails (non-zero exit), outputs stripped stderr/stdout directly
  without routing through filters (prevents error masking)

Changes

File	Change
src/yarn_cmd.rs	New module (704 lines): filter_yarn_output(), route_script(), apply_filter(),
run() + 25 tests
src/main.rs	Register Commands::Yarn with trailing_var_arg + 5 Clap parsing tests
src/lint_cmd.rs	filter_generic_lint: fn → pub(crate)
src/runner.rs	extract_test_summary: fn → pub(crate)
src/tsc_cmd.rs	filter_tsc_output: fn → pub(crate)

Test plan

• cargo fmt --all --check passes
• cargo clippy --all-targets passes
• cargo test --all passes (30+ new tests)
• Manual: rtk yarn workspace <pkg> run vitest on a real yarn workspace project
• Manual: rtk yarn workspace <pkg> run typecheck with typecheck not existing