feat(platform): inject restart context system message (#19 Layer 1)

[HongmingWang-Rabbit]

Resolves #19 partially — Layer 1 (platform-managed context) only. Layer 2 (user restart_prompt) deferred to a follow-up issue.

Change

After workspace restart and re-registration, platform sends an A2A message/send to the workspace as a synthetic user message containing:

• restart timestamp
• previous session end timestamp + duration
• env-var keys now available (keys only, no values)

Covers both the HTTP POST /workspaces/:id/restart path and the programmatic RestartByID path (auto-restart after secret change, container death, liveness timeout).

Metadata on the A2A message:
```
"metadata": { "kind": "restart_context", "source": "platform", "layer": 1, "restart_context": true }
```

Skip path

If the workspace doesn't come back online within 30s, log and drop. Restart response is unaffected.

Safety

• Values are NEVER included in the rendered message — only keys.
• System caller prefix `system:restart-context` bypasses access control and caller-token check.

Test plan

• `go build ./... && go vet ./...` pass
• `go test -race ./...` pass
• new unit tests cover humanDuration, message render (with + without prior session), and JSON-RPC payload shape
• tests/e2e/test_api.sh — skipped locally (requires running platform + Postgres); CI will run

🤖 Generated with Claude Code

[HongmingWang-Rabbit]

Merged. noteworthy: access-control-bypass — introduces system:restart-context caller prefix that bypasses CanCommunicate + A2A token validation (via existing systemCallerPrefixes allowlist in a2a_proxy.go:72). Bypass is principled (reuses existing system: scheme, not novel). 7/7 gates passed, 0 critical findings. Cross-vendor review unavailable (codex tier / gemini quota). Post-merge HEAD: 3ea8cda.