feat(agents): let goals start with context

[antoinegg1]

Summary

• Add optional context to session- and agent-level pursue / apursue.
• Send that context as an ordinary turn in the same conversation immediately before starting the backend-native goal, while keeping the goal objective unchanged.
• Cover Claude Code, Codex app-server, async forwarding, unsupported backends, and suppressed context-turn failures; document the API and its ordering semantics.

Why

Large task material is not always a suitable native goal condition. In particular, Claude Code limits a /goal condition to 4000 characters. Moving that material to a workspace file is also not portable when the coding agent's filesystem sandbox cannot initialize in an already sandboxed runtime.

Keeping the complete task in the conversation and passing only its short completion condition through the native goal interface avoids both constraints without backend-specific behavior.

Validation

• uv run pre-commit run --all-files
• uv run pytest: 1723 passed, 66 skipped
• Real FlowBench regression using the original 3728-character HLE question payload that previously produced a 4613-character /goal condition:
  • Claude Opus 5: correct structured answer, goal complete, 70.4 seconds
  • GPT-5.6 Sol through Codex app-server: correct structured answer, goal complete, 106.5 seconds
  • Codex received the full context turn followed by a 733-character goal on the same thread, with no filesystem sandbox/tool read