docs: reorganize developer documentation

crates/session-metrics/README.md

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+# session-metrics
+
+## Role
+
+`session-metrics` records usage and memory/session metrics that are useful for diagnostics and maintenance.
+
+## Boundaries
+
+Owns:
+
+- metric record types and persistence helpers
+- explicit memory usage/read/reference observations where applicable
+- lightweight diagnostic data that should not become model context by itself
+
+Does not own:
+
+- prompt context packing (`llm-worker`)
+- generated memory contents (`memory`)
+- provider billing semantics (`provider`)
+- UI status rendering (`tui`)
+
+## Design notes
+
+Metrics are observations. They may guide compaction, memory effectiveness analysis, or UX, but they are not authoritative conversation history and should not smuggle hidden state into model input.
+
+## See also
+
+- [`../../docs/design/memory-knowledge.md`](../../docs/design/memory-knowledge.md)
+- [`../../docs/design/compaction.md`](../../docs/design/compaction.md)