fix: merge platform_overrides env with base env instead of replacing

[bryan-anthropic]

Summary

• platform_overrides.*.env was replacing the entire mcp_config.env object instead of merging, causing base env vars (including ${user_config.*} references) to be silently dropped when platform overrides defined any env vars.
• One-line fix: changed result.env = platformConfig.env || result.env to spread-merge { ...result.env, ...platformConfig.env }, so platform-specific env vars augment the base env rather than replacing it.
• Added test case covering the merge behavior with both base env vars and platform override env vars.

Root Cause

In src/shared/config.ts:120, the platform override logic used simple assignment:

result.env = platformConfig.env || result.env;

When a manifest defines both mcp_config.env (e.g., LOCKSTEP_DATA_DIR: "${user_config.data_directory}") and platform_overrides.darwin.env (e.g., PATH: "..."), the darwin env completely replaced the base env, silently dropping LOCKSTEP_DATA_DIR.

This made it appear that ${user_config.*} interpolation was broken, but the variable was actually being resolved correctly — just discarded before it could reach the spawned process.

Test plan

• New test: platform override env merges with base env (preserves DATA_DIR, EXISTING_VAR, adds PATH)
• All 220 existing tests pass
• Verified with reporter's exact manifest structure — both LOCKSTEP_DATA_DIR and PATH now resolve correctly

Fixes #217

🤖 Generated with Claude Code

[claude]

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