feat: configurable root path for read-only environments

[edgett]

Summary

• MarkItDownPathResolver hardcodes .markitdown under Environment.CurrentDirectory, which is read-only in Azure Functions (and similar containerized/serverless environments), crashing on the first conversion.
• Replaced the Lazy<string> in the resolver with a lock-guarded double-check init pattern and added a Configure(string) method so the root can be redirected before any conversion runs.
• Added MarkItDownOptions.RootPath for non-DI callers and MarkItDownServiceBuilder.UseRootPath() for DI callers.

Usage

DI (Program.cs):

services.AddMarkItDown(builder =>
{
    builder.UseRootPath(Path.Combine(Path.GetTempPath(), ".markitdown"));
});

Non-DI:

var client = new MarkItDownClient(new MarkItDownOptions
{
    RootPath = Path.Combine(Path.GetTempPath(), ".markitdown")
});

Safety

• Configure() and RootPath first-access share a single lock -- no race window between writing the config and materializing the root.
• Conflicting paths throw InvalidOperationException instead of silently ignoring.
• Paths are normalized via Path.GetFullPath() with platform-aware comparison (case-insensitive on Windows, ordinal on Linux).

Files changed

• src/MarkItDown/Utilities/MarkItDownPathResolver.cs -- core fix (lock-guarded init + Configure)
• src/MarkItDown/Core/MarkItDownOptions.cs -- added RootPath property
• src/MarkItDown/Core/MarkItDownClient.cs -- 2 lines wiring RootPath in constructor
• src/MarkItDown/DependencyInjection/MarkItDownServiceBuilder.cs -- added UseRootPath() method
• AGENTS.md -- documented the configurability rule

Test plan

• Full test suite passes (383 passed, 0 failed, 5 skipped manual/live)
• Verify in Azure Function that setting RootPath to temp folder resolves the crash

---

[edgett]

I verfied:

1. Setting the new RootPath configuration option to Path.Combine(Path.GetTempPath(), "markitdown", "workspaces"); on Azure Functions, running on a Linux App Service works.
2. Setting the new RootPath configuration option to same as above works on Windows in development env.