fix(codex): recover from oversized synthesized turns

[juacker]

Summary

• budget Codex synthesized system instructions below the 1,048,576-character hard limit
• preserve the current user/task prompt verbatim and retain both ends of oversized system instructions
• add Unicode-safe regression coverage and a 4,096-character safety margin

Root cause

Context-limit recovery compacted persisted conversation history, but Codex validates the complete synthesized turn. Unbounded agent, skill, tool, and workspace instructions could dominate that turn, so compaction rebuilt essentially the same oversized input and failed again.

Verification

• cargo fmt --manifest-path src-tauri/Cargo.toml -- --check
• cargo test --manifest-path src-tauri/Cargo.toml assistant::local_agent::tests -- --nocapture (31 passed)
• independent Code Review Checklist verdict: production_quality

Note

A misconfigured upstream initially mapped the feature push to main; commit 410eb0b immediately reverted it. This PR contains the reviewed change on the explicit feature ref.