chore: extract ACP stdio transport pre-patch

[ssfdust]

ACP stdio transport pre-patch

Extracts independently-mergable fixes from the ongoing ACP work in #2858.

Background

When spawning forge machine stdio as an ACP subprocess, stdin is a pipe
that remains open for bidirectional JSON-RPC communication. This patch
resolves three classes of bug discovered during ACP integration testing.

Changes

1. stdin pre-read encapsulation (cli.rs, main.rs)
   Added Cli::uses_stdin() method that centralizes the decision of
   which subcommands own stdin. Subcommands opt in via uses_stdin()
   rather than having the startup pipeline guess from a hardcoded
   exclusion list. This makes it easy to add new stdin-consuming
   subcommands (e.g. an ACP stdio server) without touching the
   startup logic.

2. tool output leaking to stdout (context.rs, tool_executor.rs,
   chat_request.rs, orch.rs, app.rs, agent_executor.rs,
   executor.rs)
   Shell tool execution always streamed raw output to io::stdout(),
   corrupting the ACP JSON-RPC stream. Fixed in two layers:

   • Lower layer (pre-patch): Added silent: bool on ToolCallContext
     and an if silent { io::sink() } guard in the command executor.
     Default false; call sites opt in via .silent(true) to route
     output to io::sink() instead of stdout.

   • Upper layer (propagation): Added tool_silent: bool on ChatRequest
     (the per-invocation DTO) and threaded it through the entire call
     stack — ForgeApp::chat() forwards it to Orchestrator, which
     sets it on ToolCallContext. Agent-as-tool recursion inherits the
     parent's flag automatically. The propagation chain:

     ChatRequest.tool_silent → ForgeApp → Orchestrator.tool_silent → ToolCallContext.silent
                                                           ↓
                                                  tool_executor → executor.rs
                                                           ↓
                                                  agent_executor (recursive)
     

3. stderr vs stdout discipline (main.rs, sandbox.rs)
   Panic hook and worktree status messages used println!, which
   writes to stdout and contaminates structured transport. Changed
   to eprintln! (stderr).

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