feat(rewrite): translate PowerShell cmdlets to rtk subcommands

[make0uthill]

Summary

The RULES table keys off the command word, so PowerShell cmdlets — which share no name with their GNU counterparts — never match it. On Windows the agent reaches for Get-ChildItem and Select-String rather than ls and grep, so those calls reach the model as unfiltered output and the hook saves nothing.

Translates the three cmdlets whose semantics map cleanly onto an rtk subcommand, plus their common aliases (gc, gci, dir, sls):

Get-Content foo.txt -TotalCount 5      -> rtk read foo.txt --max-lines 5
Get-ChildItem src -Recurse             -> rtk ls -R src
Select-String -Path a.txt -Pattern rtk -> rtk grep -i rtk a.txt

Two details worth flagging for review:

• The -i on Select-String is not cosmetic: the cmdlet matches case-insensitively by default while grep does not, so omitting it would silently narrow the result set.
• Argument splitting cannot reuse shell_split, which treats a backslash as an escape and would turn D:\Code\x into D:Codex. ps_split keeps backslashes literal and preserves quoting.

Deliberately narrow. A cmdlet carrying any parameter without a faithful rtk equivalent (-Filter, -Raw, -File, -ErrorAction) returns None and runs unrewritten — mistranslating a flag changes what the command means, which is worse than forgoing the saving. Cmdlets are also only rewritten in RewriteContext::Normal: as a pipeline stage a cmdlet hands .NET objects to the next stage, and rtk's text output would not survive that.

Known gap, left for a follow-up: Get-ChildItem -Recurse -Filter *.rs could map faithfully onto rtk find . -name '*.rs' (and -File/-Directory onto -type f/-type d). Happy to add it here instead if you'd prefer it in one PR.

Test plan

• cargo fmt --all && cargo clippy --all-targets && cargo test — 2562 passed, 0 failed
• 8 new tests covering the translations, the pass-through cases, Windows paths, and pipeline stages
• Manual testing: verified end-to-end through rtk hook claude with a PowerShell tool payload; rewritten and passed-through cases both behave as the tests describe.

Targets develop as required.

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[make0uthill]

Pushed the follow-up: -Filter/-Include/-File/-Directory now route to rtk find instead of passing through, so the known gap noted above is closed in this PR rather than a separate one.

Get-ChildItem src -Recurse -Filter *.rs -> rtk find src -name '"*.rs"'
Get-ChildItem -Recurse -File            -> rtk find . -type f

Without -Recurse the search is depth-limited with -maxdepth 1, since Get-ChildItem only descends when asked while find recurses by default. -Include with a comma-separated list still passes through.

Full suite green, 2571 passed.

[make0uthill]

Correction to my note above: the suite is 2563 passed, 0 failed (I quoted 2563 as 2571 by mistake). Nothing else in that comment changes.