feat(observability): add gateway structured request logs

[seonghobae]

Summary

Implements #66 as the next bounded slice of #64 after the gateway SLO foundation in #65.

• adds an AsyncLocalStorage request context containing only a validated UUIDv4 correlation identifier
• adds a fixed-schema CredentialFreeJsonLogger for HTTP completion and sanitized observability-failure records
• emits exactly one completion record for normal, synchronous-failure, and client-aborted gateway requests
• preserves the bounded route inventory and collapses unknown concrete paths to /unmatched
• excludes headers, query strings, payloads, concrete identifiers, tenant data, credentials, exception messages, and stacks from the logger API
• isolates metrics, clock, context, and writer failures from the request pipeline
• adds deterministic package and gateway tests for async propagation, nested restoration, exact schemas, secret exclusion, de-duplication, aborted requests, metric failures, writer failures, and synchronous failures
• updates the gateway SLO runbook and formatting gate

Scope boundary

This slice does not implement HTTP trace-context parsing, downstream propagation, service-to-service instrumentation, OpenTelemetry exporters, managed log storage, retention automation, dashboards, or alert delivery. Those remain follow-up work in #64.

Validation gate

Merge only when formatting, lint, type checking, tests, build, AppGuardrail, SAST Semgrep, Security Scan, Commercial Readiness, CodeRabbit, and all human/security review feedback pass on the exact current head with no unresolved actionable findings.

Closes #66. Refs #64 and #21.

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📥 Commits

Reviewing files that changed from the base of the PR and between df35e53 and 2d28fb0.

📒 Files selected for processing (8)

• apps/gateway/src/observability.test.ts
• apps/gateway/src/observability.ts
• docs/operations/service-level-objectives.md
• docs/superpowers/plans/2026-08-04-gateway-structured-logging-slice.md
• package.json
• packages/observability/src/index.cjs
• packages/observability/src/index.d.ts
• packages/observability/src/index.test.cjs

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