feat(system): native Windows ls/grep/wc/tree + PowerShell cmdlet rewrites

[tienthegainz]

Summary

• Make  rtk ls / grep / wc / tree  work on Windows. These filters previously shelled out to Unix binaries ( ls ,  rg / grep ,  wc ,  tree ) that aren't present on a stock Windows install, so they failed with "Binary not found on PATH" (or  tree.com 's "Too many parameters"). Each now has a native Rust execution path used when the Unix binary is absent (detected via  tool_exists ;  tree  always goes native on Windows). All native paths reuse the existing compression filters, so token-savings behavior is unchanged.
• Auto-rewrite common PowerShell cmdlets to their rtk equivalents so the hook saves tokens for Windows/PowerShell agents:  Get-Content / gc / type  →  rtk read ,  Get-ChildItem / gci / dir  →  rtk ls  ( -Recurse  →  rtk tree ,  -Force  →  -a ),  Select-String / sls  →  rtk grep  ( -i  unless  -CaseSensitive ). Only single, non-piped invocations are rewritten so PowerShell pipelines are never broken; unmappable params (e.g.  -Filter ) and unknown cmdlets pass through untouched.
• Adds a backslash-safe PowerShell tokenizer ( ps_split ) so Windows paths like  src\cmds  survive rewriting, plus docs in  src/cmds/system/README.md .

Test plan

✓  cargo fmt --all && cargo clippy --all-targets && cargo test 
• fmt clean; clippy zero warnings; 2235 tests pass. (The 16 failing  core::stream::tests::*  are pre-existing Windows-environment failures — they spawn Unix  sh / cat / echo / sleep  — and are unrelated to this change; verified  core/stream.rs  is untouched.)
• 30 new tests added: native  wc  counts/columns,  ls  synthetic-line round-trip,  tree  arg parsing, native  grep  matching ( -i / -v , NUL-format parse round-trip), and 16 PowerShell-rewrite cases incl. backslash-path and piped-passthrough regressions.
✓ Manual testing:  rtk   output inspected on Windows (rtk 0.42.4):
•  rtk ls src\cmds\system  → compact  name size  listing + summary (was: "Binary 'ls' not found")
•  rtk grep "fn run" src\cmds\system  →  34 matches in 14 files:  grouped output, no rg/grep warnings (was: "Binary 'rg'/'grep' not found")
•  rtk wc Cargo.toml  →  72L 246W 1796B  (was: "Binary 'wc' not found")
•  rtk tree -L 1 src  → compact pruned tree (was: "Too many parameters")
•  rtk hook check --agent copilot "Get-ChildItem -Recurse src\cmds"  →  rtk tree src\cmds 
•  rtk gain --history  confirms commands tracked (~80% savings)

Important: All PRs must target the develop branch (not master).
See CONTRIBUTING.md for details.

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