feat(context): opt-in ContextSift context management

[ayman3000]

Summary

Brings the ContextSift context-management approach into SwiftAgentKit as an opt-in feature. The goal: keep long tool-heavy conversations from ballooning the prompt while never losing information.

When an AgentConfig has a ContextManager set:

• The main user/assistant conversation stays intact.
• Completed tool exchanges are externalized — each finished tool result collapses to a compact one-line receipt in a tool ledger (folded into the system prompt), and its full output is offloaded to an ArtifactStore (default InMemoryArtifactStore) so it can be retrieved losslessly on demand rather than resent every turn.
• The single in-flight (active) tool exchange is kept in full, bounded by maxActiveResultChars, so the model always has what it needs to act next.
• Two retrieval tools — artifact_read (paged) and artifact_search (line-level substring) — are auto-registered so the model can pull back any offloaded output by its artifact-… id.

Leaving contextManager as nil preserves the previous behavior exactly — makeLLMRequest routes through the manager only when one is set.

Files

New under Sources/SwiftAgentKit/Context/:

• ArtifactStore.swift — Artifact, ArtifactSlice, ArtifactMatch, protocol ArtifactStore, actor InMemoryArtifactStore.
• ToolReceipt.swift — compact receipt + ledgerLine().
• ContextManager.swift — the transformation (modelMessages(_:systemTemplate:)), receipt/artifact spill, active-exchange detection.
• ArtifactTools.swift — ArtifactReadTool, ArtifactSearchTool.

Wiring in Core/Agent.swift: AgentConfig.contextManager, tool auto-registration in Agent.init, makeLLMRequest made async and routed through the manager when present.

Tests

• Completed exchanges collapse to a ledger + artifact and drop from model context.
• Active exchange is kept in full.
• Large tool results spill to an artifact and round-trip via artifact_read.
• ArtifactStore read (paged) and search (line-level).

All 93 tests pass (swift test).

🤖 Generated with Claude Code

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