fix: clear CLAUDECODE env to prevent nested-session detection

[cyyij]

Problem

When claude-agent-acp is spawned from within an existing Claude Code session (e.g. via acpx or other agent orchestrators), the parent process sets CLAUDECODE=1. This environment variable is inherited by the spawned subprocess, causing Claude Code to detect a "nested session" and exit immediately during initialization.

The result is a generic -32603 Internal error with data.details: "Query closed before response received" on every session/new call. This affects both exec (one-shot) and persistent session modes.

Reproduction:

# From inside a Claude Code session:
acpx claude exec 'hello'
# -> [error] RUNTIME: Internal error

Root Cause

CLAUDECODE=1 is set by Claude Code in its shell environment. When claude-agent-acp spawns a new Claude Code subprocess via the SDK's query(), it passes ...process.env which includes CLAUDECODE=1. The child process sees this and refuses to start (nested-session protection).

Fix

1. Clear CLAUDECODE in subprocess env — Set CLAUDECODE: "" in the env object passed to query(), so the spawned instance starts normally. This mirrors what other Claude Code wrappers do (e.g. cmux's wrapper script uses unset CLAUDECODE).

2. Improve error handling for session/new — The v0.20.0 fix for "Query closed before response received" (PR fix: throw resourceNotFound when loadSession fails to resume #363) only covered the loadSession path (creationOpts.resume). The same error on newSession was left unhandled, producing an opaque "Internal error". Now it returns a meaningful message suggesting to check Claude Code installation.

Testing

Verified on macOS arm64 with:

• acpx 0.3.0
• Claude Code CLI 2.1.74
• Node v25.8.0

Before fix: acpx claude exec always fails with Internal error when run from within Claude Code.
After fix: works correctly, Claude responds normally.

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[cyyij]

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