feat(shell): correct known shell-dialect errors in the tool output

[trungutt]

Why

The shell tool description already tells the model which interpreter runs its commands (shellToolDescription), but observed session traces show the model still emits POSIX syntax turn after turn under Windows PowerShell — every attempt produces a fresh parse or parameter error, the model re-derives the dialect from scratch, and the same wrong syntax gets emitted again.

A representative trace across three consecutive turns in one session:

Turn 1: pwd && ls -la
        → The token '&&' is not a valid statement separator in this version.
Turn 2: docker ps | grep foo
        → grep : The term 'grep' is not recognized as the name of a cmdlet...
Turn 3: docker logs foo | tail -20
        → tail : The term 'tail' is not recognized as the name of a cmdlet...

The description hint prevents turn 1 in some sessions but does not recover the sessions where the model ignores it. Long agentic sessions burn many tool calls this way.

What changes for the model

Scan the shell-tool output for well-known dialect-error signatures and prepend a terse imperative hint above the raw output:

[shell-hint] You are on PowerShell: POSIX utilities are not available.
Use `Select-String` (grep), `Select-Object -First N` (head), ...

grep : The term 'grep' is not recognized as the name of a cmdlet...

The hint lands at the top so the model sees it first; the original output is preserved verbatim below so nothing about the debugging path is hidden.

Why gated on the resolved shell, not on a raw substring match

Without gating, a docker logs pull from a Windows container, a grep against a repo, or a cat of a docs page that quotes the error string would all trip false positives on macOS/Linux hosts. Gating also makes each hint truthful — a "chain with ; not &&" nudge is actively wrong when the resolved shell is PowerShell 7 (pwsh) or when the failure came from a child cmd.exe invocation under a PowerShell parent.

[aheritier]

👋 This PR has merge conflicts with the base branch. Please rebase or merge the latest base branch and resolve them. I've moved it to draft and added status/needs-rebase; it'll be picked back up automatically once the conflicts are cleared.