Add hosted-safe protocol passthrough with injectable I/O context

[carldebilly]

Why

AsProtocolPassthrough() keeps stdout clean for stdio protocols (MCP/LSP/DAP/CGI-style), but hosted sessions have a different stream ownership model.

This PR adds an explicit low-level I/O contract for handlers and makes passthrough behavior safe and predictable in hosted transports.

What changed

• Added new injectable service: IReplIoContext
• Added default implementation: LiveReplIoContext
• Registered IReplIoContext in core/default DI paths (CLI and hosted overlay)
• Extended protocol passthrough runtime behavior:
  • Local CLI: unchanged (stdout reserved for protocol stream, repl diagnostics to stderr)
  • Hosted session: fail fast unless handler explicitly requests IReplIoContext
• Added explicit runtime error code for unsupported hosted passthrough:
  • protocol_passthrough_hosted_not_supported
• Marked IReplIoContext as framework-injected for documentation export (not exposed as user option)
• Updated docs:
  • new docs/commands.md
  • README passthrough guidance
  • shell completion notes preserved

Developer-facing behavior

A hosted passthrough handler must be stream-capable, e.g.:

app.Map("transfer send", (IReplIoContext io) =>
{
    io.Output.WriteLine("zmodem-start");
    return Results.Exit(0);
})
.AsProtocolPassthrough();

Console-bound libraries that open Console.* directly remain CLI-only.

Tests

Added/updated coverage for:

• CommandBuilder passthrough API and shell completion bridge marking
• passthrough banner suppression and stdout/stderr cleanliness
• hosted fail-fast when passthrough handler is not stream-capable
• hosted success when handler requests IReplIoContext
• IReplIoContext injection in CLI mode

Validated with:

• Repl.Tests ✅
• Repl.IntegrationTests ✅
• Repl.ProtocolTests ✅

Compatibility

• No breaking change to AsProtocolPassthrough() API surface
• Behavioral change is scoped to hosted sessions:
  • previously ambiguous
  • now explicit fail-fast unless stream-capable handler is declared

Notes

This lays groundwork for niche remote stream workflows (for example ZMODEM-like transfers) without weakening safety for console-bound protocol SDKs.