refactor!: simplify identity model; drop acting-user plumbing

action.yaml

@@ -27,14 +27,6 @@ inputs:
     description: "GitHub token used to post and update issue comments."
     required: true
 
-  github-user-id:
-    description: "GitHub user ID to resolve to a Coder user. Mutually exclusive with coder-username."
-    required: false
-
-  coder-username:
-    description: "Coder username to use directly. Mutually exclusive with github-user-id; useful for service-account workflows."
-    required: false
-
   coder-organization:
     description: "Coder organization name. Looked up by name to resolve the organization UUID for chat creation. Recommended when the resolved Coder user belongs to more than one organization, since the fallback choice is non-deterministic."
     required: false
@@ -67,7 +59,7 @@ inputs:
     default: "600"
 
   idempotency-key:
-    description: "Optional sharding key to narrow the default per-workflow scope. By default the action reuses the most recent non-archived chat scoped to `gh-target`, the resolved Coder user, and the workflow name. Set this to maintain multiple parallel chats on the same target/user/workflow (for example, one per matrix dimension)."
+    description: "Optional sharding key to narrow the default reuse scope. By default the action reuses the most recent non-archived chat scoped to `gh-target` and the workflow name (when `GITHUB_WORKFLOW` is set). All chats are owned by the `coder-token` holder, so the scope does not include a per-actor component. Set this to maintain multiple parallel chats on the same target/workflow (for example, one per matrix dimension). Pass `${{ github.actor }}` here if you want per-actor separation."
     required: false
 
   force-new-chat:
@@ -77,7 +69,7 @@ inputs:
 
 outputs:
   coder-username:
-    description: "The Coder username resolved from the GitHub user."
+    description: "The Coder username the `coder-token` belongs to (always the chat owner; the chats API has no owner override)."
 
   chat-id:
     description: "The chat ID."
@@ -125,7 +117,7 @@ outputs:
     description: "Base branch name when available."
 
   chat-error-kind:
-    description: "Machine-readable error kind when the chat fails (one of `spend_exceeded`, `user_not_found`, `user_ambiguous`, `org_not_found`, `api_error`, `timeout`)."
+    description: "Machine-readable error kind when the chat fails (one of `spend_exceeded`, `org_not_found`, `api_error`, `timeout`)."
 
   chat-error-message:
     description: "Human-readable error message when the chat fails."