fix: standardize API error response envelope

[OneStepAt4time]

Summary

• add centralized JSON error-envelope normalizer for API responses
• include standard fields: code, message, optional details, requestId
• preserve backward compatibility by keeping legacy error field
• wire normalizer once in server onSend hook (no per-route rewrites)
• skip normalization for SSE payloads to avoid stream behavior changes

Closes #399

Test Evidence

• npx tsc --noEmit
• npm test -- src/tests/api-error-envelope.test.ts
• npm run build

[aegis-gh-agent]

Code review passed — centralized error envelope is clean. Approved.

[aegis-gh-agent]

Self-rebase failed — conflicts with merged release 2.8.0 across .release-please-manifest.json, CHANGELOG.md, package.json, package-lock.json, src/server.ts. Please rebase onto current main and force-push. Approved — will merge immediately on green CI.

[aegis-gh-agent]

Self-rebase failed — conflicts with merged PR #951 (MemoryBridge) across package.json, package-lock.json, .release-please-manifest.json, CHANGELOG.md, src/server.ts. Please rebase onto current main and force-push. Approved — will merge immediately on green CI.

[aegis-gh-agent]

Centralized error envelope normalizer via onSend hook is clean — SSE skip, backward compatible, requestId tracing. Approved.

[aegis-gh-agent]

Self-rebase failed — conflicts with release PR #941 across package.json, package-lock.json, CHANGELOG.md, and src/server.ts. Please rebase and resolve, keeping only the server.ts changes (discard package/changelog noise from the merge). Approved — will merge on green CI.

[aegis-gh-agent]

Standardized error envelope. Centralized normalizer in onSend hook, preserves legacy error field, skips SSE. Tests cover envelope fields and SSE bypass. Approved.