fix: replace REACH_SESSION in harness environments rather than appending it

[bojieli]

What this changes

Every harness launcher built its child environment as os.Environ() plus "REACH_SESSION=<name>". When the operator's shell already exported REACH_SESSION — the documented way to bind a shell to a session — the child received the key twice, and which of the two a program reads is not defined.

The failure that matters: the shim would resolve a different session than the launch had just named on stderr, so the agent's commands would run on that session's target while every message said this one. That is silent wrong-target execution, arrived at through reach's own environment handling.

replaceEnv already existed for exactly this reason and was already used for CODEX_HOME and HOME. It is now used for REACH_SESSION in every launcher, and for CLAUDE_CODE_SHELL_PREFIX, which had the same shape.

The harness seam probes had the same bug with worse consequences: baseProbeEnv and claudeCodeEnv copy the current environment and append the throwaway probe session, so an inherited REACH_SESSION could win and route the probe's canary through the operator's real session. The probe would then return a verdict about a seam it never exercised — the one thing a fail-closed guard must never do. Both now strip an inherited value before setting their own.

Checklist

• make check and make lint pass
• make integration passes, if this touches transports or file operations — not run; this touches neither
• Comments explain why where the code looks odd

If this touches a harness adapter:

• A conformance test covers the seam, and fails when the seam changes shape — no seam shape changed; this is environment construction on the launcher side
• docs/RESEARCH.md records what was observed — no new harness observation
• Anything not verified is written down as unverified rather than implied to work

Verified by unit tests and make check only: reproducing the duplicate-key path end to end needs a real harness and a target, and was not run here.

🤖 Generated with Claude Code

https://claude.ai/code/session_018WMXbmmHmEkyWWRc72uvPW