[FEATURE]: Propagate session ID to tool subprocesses via environment variable

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Describe the enhancement you want to request

Feature Request

Problem

When OpenCode executes a tool (e.g., a bash command), it spawns a subprocess. That subprocess has no way to discover which OpenCode session triggered it. This makes it impossible for external systems — or even
the tool itself — to correlate their work back to a specific session.

Use Case

If you build tooling that wraps or extends OpenCode's tool execution (e.g., logging, metrics, multi-session orchestration), you need to know which session a tool invocation belongs to. Today, the only way to
attempt this is fragile heuristics like FIFO matching or content-based comparison, because the subprocess environment contains no session context.

Proposed Solution

Set environment variables on tool subprocesses before they are spawned:

• OPENCODE_SESSION_ID — the session that triggered the tool call
• OPENCODE_TOOL_PART_ID (optional) — the part ID of the tool invocation within the message, for finer-grained correlation

This is a minimal, non-breaking change. Tools that don't need the information simply ignore the env vars.

Precedent

PR #12932 already propagates session context (as HTTP headers) for LLM requests. This proposal extends the same pattern to tool subprocess execution.