[codex] Add logging and tracing guide

[samlaycock]

Summary

• Add a focused docs/logging-tracing.md guide covering fx.log, fx.logWarn, fx.logError, fx.span, and fx.trace.
• Document structured log metadata, span attributes, span options, and how these helpers map to native Effect primitives.
• Link the new guide from docs/README.md, tighten the API reference wording, add a README example, and include a patch changeset.

Why

Issue #35 notes that logging and tracing helpers are public API but only listed in the API reference. This PR gives users a focused behavior guide so they do not need to infer metadata and span semantics from source or Effect docs.

Validation

• bun fmt
• bun lint:fix
• bun lint
• bun typecheck
• bun test

Closes #35.

[greptile-apps]

Greptile Summary

This PR adds a focused docs/logging-tracing.md guide for fx.log, fx.logWarn, fx.logError, fx.span, and fx.trace, then links it from the docs index, README, and API reference. The documented signatures were verified against src/logging.ts and are accurate.

• New guide (docs/logging-tracing.md): covers structured log metadata, span attributes, Tracer.SpanOptions, the mapping to native Effect primitives, and when to reach for fluent-effect/effect directly.
• README / docs index: heading updated to "Retry, Timeout, Logging, Tracing", fx.log example added, and the new guide linked in both README.md and docs/README.md.
• Changeset included as required by project conventions.

Confidence Score: 5/5

Documentation-only change; no runtime code is modified.

All changes are prose and Markdown. The documented API signatures match the source implementation exactly. No logic, types, or runtime behaviour are altered.

No files require special attention.

Important Files Changed

Filename	Overview
docs/logging-tracing.md	New guide covering fx.log/logWarn/logError and fx.span/trace; API signatures verified against src/logging.ts. One code block is missing the ts language tag.
README.md	Section heading updated and fx.log example added; consistent with the new guide and existing examples.
docs/README.md	New guide linked correctly in the docs index.
docs/api-reference.md	API table descriptions tightened to mention structured metadata and span options.
.changeset/fair-pandas-trace.md	Patch changeset added as required by AGENTS.md conventions.

_{Reviews (2): Last reviewed commit: "docs: fix logging example yield syntax" | Re-trigger Greptile}