feat: Update secrets mcp tool

docs/vantage/mcp.md

@@ -120,6 +120,8 @@ surface is a presentation layer over existing authority, not a new one.
 | `create_step` | Create a reusable workflow step. | `mcp:write`, `workflows:write` |
 | `create_workflow` | Create a workflow from existing step IDs. | `mcp:write`, `workflows:write` |
 | `create_monitor` | Create a monitor, saved disabled. Supports every type, including `heartbeat` (the ping token is returned once) and `metric`. | `mcp:write`, `monitors:write` |
+| `create_secret` | Add new keys to a secret group, creating the group if needed. Refused if any key already exists. | `mcp:write`, `secrets:write` |
+| `update_secret` | Replace the values of existing secret keys. Refused if any key does not exist. Values are never returned. | `mcp:write`, `secrets:write` |
 
 A known limitation worth calling out on `search_fleet`: its `version_below`
 argument is not implemented. Passing it gets you an error explaining that
@@ -170,7 +172,11 @@ tell at a glance which scripts came from an agent rather than a person.
 
 There is no tool to update or delete an existing step, workflow or monitor.
 An agent can add to your instance; it cannot change or remove anything that
-was already there.
+was already there, with one exception: `update_secret` replaces the value of
+an existing secret key. Anything using that key gets the new value from then
+on, so a token with `mcp:write` and `secrets:write` can break whatever relies
+on the old one. The change is recorded in the audit log as `mcp.updated`, with
+the key names but not the values. No tool deletes a secret.
 
 ## What an agent cannot do