fix: tell models the workspace root so filesystem tools work on remote setups

[bunnysayzz]

Description

Fixes #13082 — Continue Agent filesystem tools resolve C:\workspace instead of the actual workspace, breaking all filesystem tools on Remote-SSH (and Dev Containers / any vscode-remote:// setup).

Root cause: the agent/plan system message instructs models to use filesystem tools with paths "relative to the root of the workspace" — but never tells the model what that root is. On local workspaces the model can infer it from file:// context URIs, but on Remote-SSH the workspace is a vscode-remote://ssh-remote+host/opt URI, so no context ever reveals the real path. The model then guesses (C:\workspace), and every readFile/ls/viewSubdirectory call fails with "The directory c:\workspace does not exist."

Fix: when the session has filesystem tools available, inject a <workspace_info> block into the agent/plan system message stating the resolved workspace root:

<workspace_info>
Your workspace root is: /opt/billing
When using filesystem tools, pass paths relative to this root (e.g. "src/main.ts" for /opt/billing/src/main.ts) or absolute paths. Explore the workspace with the viewSubdirectory or ls tools instead of guessing paths.
</workspace_info>

The root is derived from window.workspacePaths[0] (the same source the session metadata already uses) via a new getWorkspaceDisplayPath() helper that handles:

• file:///C:/Users/... → C:/Users/... (Windows local)
• file:///home/user/proj → /home/user/proj (POSIX local)
• vscode-remote://ssh-remote+host/opt → /opt (Remote-SSH / Dev Containers)
• vscode-vfs://... → remote path
• untitled:/empty → skipped (no injection)

Chat mode is untouched; the injection only happens in agent/plan modes when tools are actually enabled, so it never contradicts the no-tools warning.

Checklist

• I have read the Contributing Guidelines
• I have read the Code of Conduct
• I have read and agree to the Contributor License Agreement
• I have tested the changes locally
• I have added tests covering the changes (getWorkspaceDisplayPath unit tests + system-message injection tests)

Testing

• vitest run src/redux/util/ — 46/46 tests pass (3 new test cases covering the helper and the injection)
• tsc --noEmit — clean on all changed files (pre-existing workspace-package errors in core/ are unrelated)
• Manually verified the helper output for Remote-SSH, Dev Containers, vscode-vfs, local POSIX, local Windows, and unsupported schemes

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[bunnysayzz]

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