* fix: strip skill-injected env vars from ACP harness spawn env
Skill apiKey entries (e.g., openai-image-gen with primaryEnv=OPENAI_API_KEY)
are set on process.env during agent runs and only reverted after the run
completes. ACP harnesses like Codex CLI inherit these vars, causing them
to silently use API billing instead of their own auth (e.g., OAuth).
The fix tracks which env vars are actively injected by skill overrides in
a module-level Set (activeSkillEnvKeys) and strips them in
resolveAcpClientSpawnEnv() before spawning ACP child processes.
Fixes#36280
* ACP: type spawn env for stripped keys
* Skills: cover active env key lifecycle
* Changelog: note ACP skill env isolation
* ACP: preserve shell marker after env stripping
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Co-authored-by: Vincent Koc <vincentkoc@ieee.org>
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