[enhancement] E5-2: No API versioning headers — no deprecation path

[OneStepAt4time]

Finding

ID: API-2 | Severity: 🟡 MEDIUM

No version negotiation, no deprecation headers. URL prefix /v1/ is the only version signal.

Fix Required

1. Add X-Aegis-API-Version: 1 response header to all /v1/ responses
2. Accept Accept: application/vnd.aegis.v1+json request header and validate
3. Document deprecation path for future /v2/ transition

Files: src/server.ts

Acceptance Criteria

Every /v1/ response includes X-Aegis-API-Version: 1.

Milestone

M-E5: API & Integration

[OneStepAt4time]

Implemented in PR #1524 (merged). X-Aegis-API-Version: 1 header added to all /v1/ routes.