FDE-45: Add automatic log-trace correlation for slog

[prathamesh-sonpatki]

Summary

• Adds instrumentation/slog package that wraps any slog.Handler to automatically inject trace_id and span_id from OTel span context into log records
• Enables log-trace correlation without requiring users to set up an OTel LoggerProvider — logs stay in the existing pipeline (stdout/file)
• Follows existing instrumentation package conventions (gin, echo, nethttp, etc.)

Key Design Decisions

• Attribute injection approach over OTel Log Bridge — lightweight, no new OTel providers needed
• r.Clone() before AddAttrs — prevents data races on the slog.Record's internal slice
• sc.IsValid() gate — injects even for sampled-out spans (valid IDs still useful for correlation)
• Options struct — allows custom attribute key names (e.g., dd.trace_id for Datadog)
• No agent auto-start dependency — handler reads span context directly from ctx, works independently

API Surface

slogagent.NewHandler(inner slog.Handler, opts *Options) *Handler
slogagent.NewJSONHandler(w io.Writer, handlerOpts, opts) *Handler
slogagent.NewTextHandler(w io.Writer, handlerOpts, opts) *Handler
slogagent.SetDefault(w io.Writer, handlerOpts, opts) *slog.Logger

Test Plan

• No span → no injection (13 tests, all passing)
• Active span → injects 32-char trace_id + 16-char span_id
• Sampled-out span → still injects (sc.IsValid() == true)
• Remote span context → injects correctly
• Custom keys via Options
• WithAttrs/WithGroup preserve trace injection
• Enabled delegates to inner handler
• Nil inner panics with clear message
• SetDefault sets global logger
• User attributes preserved alongside trace attrs
• golangci-lint clean

Linear: https://linear.app/last9/issue/FDE-45

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