Go: Phase 6 — Compaction

[juliomenendez]

Summary

• CompactionStrategy interface with Compact(ctx, messages) ([]Message, bool, error)
• Tokenizer interface + CharEstimatorTokenizer (1 token per 4 chars)
• TruncationStrategy — keeps last N non-system messages, always preserves system messages
• SlidingWindowStrategy — drops oldest non-system messages until under token budget
• SummarizationStrategy — uses a ChatClient to summarize older messages, keeps recent N untouched
• PipelineStrategy — chains multiple strategies, stops at first that compacts
• WithCompactionStrategy AgentOption — compaction runs in runCore before LLM call, doesn't affect history storage

Test plan

• go test ./... -count=1 — all tests pass across 4 packages
• go vet ./... — no issues
• go mod tidy — module is clean
• CharEstimatorTokenizer tests
• TruncationStrategy tests (compaction, no-op, system-only)
• SlidingWindowStrategy tests (token budget, no-op, system preservation)
• SummarizationStrategy tests (LLM summary, no-op under budget)
• PipelineStrategy tests (chain execution, no-op)
• Agent integration tests (compaction reduces messages, no strategy = all messages)

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