feat(agent): context-aware auto-compaction with retry on context_window_exceeded

[Rexopia]

Background

Currently auto_compact_history triggers based on message count only (max_history_messages, default 200). This is a rough proxy for context size — it doesn't account for actual token usage or context window limits.

Current Behavior

1. auto_compact_history runs once at the start of each turn, compacting old messages into an LLM-generated summary when message count exceeds the limit
2. reliable.rs already detects context_window_exceeded errors via is_context_window_exceeded() (matching keywords like "exceeds the context window", "maximum context length", "too many tokens", etc.)
3. But on detection, it bail!s immediately — no recovery, no retry

Proposed Improvement

Phase 1: Compact-then-retry on context exceeded

In orchestrate.rs, when the LLM call returns a context_window_exceeded error:

1. Force-run auto_compact_history (even if message count is within limit)
2. Retry the LLM call
3. If still fails, then fatal error

This is the minimal useful change — leverage the existing error detection in reliable.rs as a reactive trigger.

Phase 2 (future): Token-based proactive compaction

• Track cumulative input_tokens from provider responses across turns
• Store per-model context window sizes (from registry or model metadata)
• Trigger compaction when input_tokens > context_window * 0.8
• Requires providers to reliably return token usage (Codex SSE may not always include it)

Reference

• crewforge-rs/src/agent/orchestrate.rs:274-287 — current fatal error on LLM failure
• crewforge-rs/src/agent/history.rs:114-172 — auto_compact_history implementation
• crewforge-rs/src/provider/reliable.rs:82-88 — is_context_window_exceeded detection
• Upstream (vendor/zeroclaw) uses the same message-count approach, no token-based compaction either

Related Constants

• COMPACTION_KEEP_RECENT_MESSAGES = 20
• COMPACTION_MAX_SOURCE_CHARS = 12,000
• COMPACTION_MAX_SUMMARY_CHARS = 2,000
• Token estimation heuristic: ~4 chars/token (from upstream)