observability.md lists span attributes the wrapper never emits #51

Category: docs Severity: major
Location: docs/guides/observability.md:69-72

What

Same root cause as the Arcp.Otel.md table: ArcpTracing.SetCommonAttributes (src/Arcp.Otel/ArcpTracing.cs:82-92) sets seven tags and never sets arcp.agent/arcp.lease.capabilities/arcp.lease.expires_at/arcp.budget.remaining (grep of all SetTag calls confirms).

Evidence

| `arcp.agent`               | `payload.agent` (on submit / accept)          |
| `arcp.lease.capabilities`  | comma-joined lease keys                       |
| `arcp.lease.expires_at`    | ISO 8601 string (v1.1)                        |
| `arcp.budget.remaining`    | JSON-stringified currency map (v1.1)          |

Proposed fix

Remove the four unimplemented rows, or implement the tagging in ArcpTracing.

Acceptance criteria

• The table reflects only attributes ArcpTracing actually sets.