feat(spawn): pass Codex startup prompts as developer instructions

[laynepenney]

Summary

• load each configured Codex startup_prompt and pass it through native developer_instructions
• keep recall continuity separate as the optional first user payload
• fail closed when a configured Codex prompt cannot be read
• submit pasted Codex payloads with the current three-Enter TUI sequence
• atomically replace generated launch scripts as owner-only files so elevated prompts are never written to a readable inode
• document runtime-specific startup prompt semantics

Verification

• cargo fmt --check
• cargo test --lib (739 passed)
• cargo check --all-targets
• installed Codex prompt-input probe with the 54,543-byte Stromus prompt, confirming identity at developer role and the actual turn at user role
• Sentinel R4 independent gate: RATIFY, no remaining findings

Linked continuity rollout

• perf: bound session reads and package continuity hooks recall#1002
• synapt-dev/config#538
• synapt-dev/gripspace#16