fix(agent): Preserve Codex base prompt via developer_instructions

[charlesvien]

Problem

Codex received PostHog's guidance through the instructions config key, which replaces Codex's model-optimized base prompt instead of appending to it.

Changes

1. Pass guidance via Codex developer_instructions (appends) instead of instructions (replaces the base prompt)
2. Rename the instructions option to developerInstructions across the agent and workspace-server call sites
3. Add a spawn test asserting guidance uses developer_instructions, never the replacing key

How did you test this?

Manually

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

[charlesvien]

• feat(agent): Default Codex to the native app-server harness #2723
• fix(agent): Send cloud Claude reasoning effort without plugins #2722
• fix(agent): Preserve Codex base prompt via developer_instructions #2721 👈 (View in Graphite)
• main

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[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit ad7af09.}

[greptile-apps]

_{Reviews (1): Last reviewed commit: "append codex guidance via developer_inst..." | Re-trigger Greptile}

[github-actions]

Consistent internal rename of instructions to developerInstructions across the Codex adapter chain, with matching tests that verify the new subprocess config key and guard against the old ones. No external API contracts, security issues, or broken references.