feat(security): A-1 Ed25519 key rotation + A-2 replay attack fixes

Complete RedFlag codebase with two major security audit implementations.

== A-1: Ed25519 Key Rotation Support ==

Server:
- SignCommand sets SignedAt timestamp and KeyID on every signature
- signing_keys database table (migration 020) for multi-key rotation
- InitializePrimaryKey registers active key at startup
- /api/v1/public-keys endpoint for rotation-aware agents
- SigningKeyQueries for key lifecycle management

Agent:
- Key-ID-aware verification via CheckKeyRotation
- FetchAndCacheAllActiveKeys for rotation pre-caching
- Cache metadata with TTL and staleness fallback
- SecurityLogger events for key rotation and command signing

== A-2: Replay Attack Fixes (F-1 through F-7) ==

F-5 CRITICAL - RetryCommand now signs via signAndCreateCommand
F-1 HIGH     - v3 format: "{agent_id}:{cmd_id}:{type}:{hash}:{ts}"
F-7 HIGH     - Migration 026: expires_at column with partial index
F-6 HIGH     - GetPendingCommands/GetStuckCommands filter by expires_at
F-2 HIGH     - Agent-side executedIDs dedup map with cleanup
F-4 HIGH     - commandMaxAge reduced from 24h to 4h
F-3 CRITICAL - Old-format commands rejected after 48h via CreatedAt

Verification fixes: migration idempotency (ETHOS #4), log format
compliance (ETHOS #1), stale comments updated.

All 24 tests passing. Docker --no-cache build verified.
See docs/ for full audit reports and deviation log (DEV-001 to DEV-019).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

aggregator-server/.env.example

@@ -0,0 +1,12 @@
+# Server Configuration
+SERVER_PORT=8080
+
+# Database Configuration
+DATABASE_URL=postgres://aggregator:aggregator@localhost:5432/aggregator?sslmode=disable
+
+# JWT Secret (CHANGE IN PRODUCTION!)
+JWT_SECRET=change-me-in-production-use-long-random-string
+
+# Agent Configuration
+CHECK_IN_INTERVAL=300          # seconds
+OFFLINE_THRESHOLD=600          # seconds before marking agent offline