feat(audit): stamp entity_id + entity_type on every event (closes #164)

[initializ-mk]

Summary

Adds top-level `entity_id` and `entity_type` fields to every Forge audit event, sourced from `FORGE_AGENT_ID` (or forge.yaml `agent_id`) with `entity_type` hardcoded to `"agent"`. Field names + values match the guardrails library's vocabulary 1:1 so consumers can join the Forge NDJSON stream against the library's MongoDB `GuardrailAuditEvent` collection on `(entity_id, entity_type)` without translation.

Why entity_id + entity_type, not agent_id

Original #164 proposal said `agent_id`. Renamed to `entity_id` + `entity_type` for two reasons:

1. 1:1 column compatibility — The guardrails library's `BasePayload` carries `EntityID` + `EntityType` (constants `agent` / `workflow` / `assistant`). When `FORGE_GUARDRAILS_DB` is set, the library writes `GuardrailAuditEvent` records into MongoDB with these exact column names. Forge using `agent_id` would force every consumer reading both streams to maintain a translation table forever.

2. Future-proof for non-agent entities — Forge runs agents today, but the library already supports workflows and assistants. Encoding the entity type as a value (not a field name) means adding a second entity type later is an additive value change, not a schema change.

Precedence

Two layers — no per-request header layer like #157 has, because entity identity is fixed at process startup:

Layer	Source	Wins when
1 — Explicit on event	`AuditEvent.EntityID` / `AuditEvent.EntityType`	Always
2 — Deployment-time stamp	`FORGE_AGENT_ID` env → `cfg.AgentID` (forge.yaml); `entity_type` hardcoded to `"agent"`	Whenever the higher layer is empty

If a deployment needs per-request entity routing, the tenancy layer (`X-Forge-Org-ID` / `X-Forge-Workspace-ID`) from #157 already covers that — agent identity is the process, by definition.

Wiring (forge-cli/runtime/runner.go)

Mirrors `BuildGuardrailChecker`'s existing AgentID resolution at `guardrails_loader.go:46-50` — env wins over forge.yaml. Called right after the existing `WithTenancy` stamp so all four tenancy/entity fields land together on every emit:

```go
agentID := os.Getenv("FORGE_AGENT_ID")
if agentID == "" && r.cfg.Config != nil {
agentID = r.cfg.Config.AgentID
}
auditLogger.WithEntity("agent", agentID)
```

Event shape

Before:
```json
{"ts":"...","event":"session_start","correlation_id":"...","task_id":"...","org_id":"org_x","workspace_id":"ws_y"}
```

After (no behavior change unless `FORGE_AGENT_ID` or forge.yaml `agent_id` is set):
```json
{"ts":"...","event":"session_start","correlation_id":"...","task_id":"...","org_id":"org_x","workspace_id":"ws_y","entity_id":"my-agent","entity_type":"agent"}
```

Files

File	Change
`forge-core/runtime/audit.go`	`EntityID` / `EntityType` fields on `AuditEvent` (omitempty); `tenantEntityID` / `tenantEntityType` on `AuditLogger`; `WithEntity(entityType, entityID)` setter; `entityStamp()` internal accessor; stamp pass in `Emit` (no ctx layer needed; `EmitFromContext` reaches `Emit` at the end)
`forge-cli/runtime/runner.go`	Read `FORGE_AGENT_ID` / `cfg.AgentID`, call `auditLogger.WithEntity(...)` right after the existing `WithTenancy` call
`forge-core/runtime/entity_test.go` (new)	Static stamp on plain Emit, omit when unset, EmitFromContext per-invocation events, explicit value beats stamp, partial WithEntity
`docs/security/audit-logging.md`	New Entity stamping subsection with the 1:1 library-join note
`docs/security/tenancy.md`	Precedence table split into Tenancy fields + Entity fields subsections; documents the no-header-layer choice

Test plan

• `go test ./...` clean in forge-core and forge-cli
• `golangci-lint run ./...` → 0 issues in both modules
• `gofmt -w` applied
• All 5 new unit tests pass: `TestAuditLogger_StaticEntityStampsPlainEmit`, `TestAuditLogger_NoEntityStamp_OmitsFields`, `TestEmitFromContext_StaticEntityStampsPerInvocationEvents`, `TestEmitFromContext_ExplicitEntityValueWins`, `TestAuditLogger_WithEntity_PartialStamp`
• End-to-end smoke: run with `FORGE_AGENT_ID=aibuilderdemo`, tail audit socket, confirm every event (startup banners + per-invocation rows) carries `"entity_id":"aibuilderdemo","entity_type":"agent"`
• With `FORGE_GUARDRAILS_DB` set: confirm the library's `GuardrailAuditEvent.entity_id` value matches Forge's NDJSON `entity_id` value

Schema impact

Additive only. Both keys use `omitempty`. Deployments setting neither env nor forge.yaml `agent_id` keep emitting the pre-#164 JSON shape verbatim — no `AuditSchemaVersion` bump.

Stamping stack now complete

After this merges, every audit event carries the full deploy identifier set:

Field	Source	PR
`org_id`	env + per-request header	#157
`workspace_id`	env + per-request header	#157
`entity_id`	env + forge.yaml	this PR
`entity_type`	hardcoded `"agent"` (future-extensible)	this PR

SIEM filter: `org_id=X AND workspace_id=Y AND entity_id=Z` uniquely identifies a Forge deploy across the export stream.

Closes #164