fix: redesign compaction with tree branching and add thinking support

[urmzd]

Summary

• Redesign compaction to use tree branching: instead of mutating history in-place, compaction now forks a new branch off the tree root with summarized history, preserving the original branch untouched
• Add thinking support to Ollama generation (falls back to thinking content when response is empty)
• Fix sliding window compactor to preserve tool-call/tool-result pairs
• Add configurable KeepLast parameter to SummarizeCompactor
• Replace custom joinStrings with strings.Join in compressing assembler
• Remove unused WithStore tree option

Test plan

• Added persistCompacted unit tests (branch creation, too-short error)
• Added tryCompact unit tests (no compactor, below threshold, success)
• Added sliding window tool-pair integrity tests
• Added SummarizeCompactor KeepLast configuration tests
• Added MessagesToText tool use content tests
• Updated existing integration test to verify branch forking behavior