feat(discover): classify PowerShell cmdlets

[rhishi99]

Problem

The rewrite registry only knows POSIX command names. On the default Windows shell almost nothing classifies — Get-ChildItem, Get-Content and Select-String are the everyday equivalents of ls, cat and grep, and all three pass through unrecognized. A Windows user gets close to zero rewrites on the commands they type most.

Fix

Rules for those cmdlets and their standard aliases (gci, dir, gc, sls, …), each mapped to the rtk command that already handles it.

Ordering is the whole trick

classify_command takes matches.last(), so a specific pattern placed before the broader one it refines never wins. Written the natural way — specific first — this happens:

Command	Resolves to	Should be
Get-ChildItem -Recurse src	rtk ls	rtk tree
Get-ChildItem Env:	rtk ls	rtk env
Get-Content -Head 50 f.txt	(dead rule)	rtk read at 60%

General rules go first, specific ones last. The tests assert the resolution, not just that a pattern matches, so this ordering cannot silently regress.

Two rules deliberately dropped

• Test-Path → rtk test-path — that command does not exist. Verified: rtk test-path . prints Failed to resolve 'test-path' via PATH ... [rtk: program not found]. It would have replaced a working user command with an error.
• Get-Location/pwd → rtk pwd at a claimed 30% saving — the output is one line, and bare pwd is already in IGNORED_PREFIXES.

Testing

Seven new classification tests in registry.rs covering each cmdlet, alias equivalence (gci ≡ Get-ChildItem), and the three ordering cases in the table above.

Full suite on x86_64-pc-windows-gnu: 2622 passed. cargo fmt --check and cargo clippy --all-targets clean. The single unrelated failure on this toolchain is fixed in #3615.

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