feat: typed Metrics interface and auto-detected client version

[inhuman]

Summary

• Metrics interface — RecordCall(server, tool string, dur time.Duration, err error) and RecordToolList(server string, count int). Register via WithMetrics(m Metrics); passing nil is a no-op. Panics inside any method are recovered by the library. Works alongside existing AfterCallHook.
• RecordCall is invoked after every CallTool invocation including pre-RPC failures (ErrToolNotFound, etc.). dur measures the upstream MCP call wall-clock time; near-zero for pre-RPC failures.
• RecordToolList is invoked after every successful tool-list fetch (initial connect + live refresh).
• Auto-detected clientInfo.version — reads the consuming module's version via runtime/debug.ReadBuildInfo(); falls back to "dev" for go run / untagged builds. WithClientIdentity continues to override.
• Zero new external dependencies (runtime/debug is stdlib).

Test plan

• TestMetrics_RecordCall_Success — RecordCall fires with nil err and dur > 0
• TestMetrics_RecordCall_Error — RecordCall fires with non-nil err for ErrToolNotFound
• TestMetrics_RecordToolList — RecordToolList fires on connect with correct count
• TestMetrics_PanicRecovered — panic in Metrics doesn't propagate to caller
• TestMetrics_NilNoOp — WithMetrics(nil) is safe
• TestMetrics_WithAfterCallCoexists — both Metrics and AfterCallHook fire
• TestVersion_DefaultIsDev — defaultClientVersion returns "dev" in test env
• TestVersion_WithClientIdentityOverrides — explicit override takes precedence
• CI: go test -race ./..., golangci-lint run — all green

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