docs(action.yaml): backtick github.actor and use canonical context terminology (#34)

On `action.yaml:62`, wrap `github.actor` in backticks for consistency with the surrounding identifiers (`gh-target`, `GITHUB_WORKFLOW`, `coder-token`) and replace "workflow expression" with the canonical GitHub Actions term "context value", per the [R2 review on #33](#33 (review)).

🤖 Authored by Coder Agents.

Author: mafredri

action.yaml

@@ -59,7 +59,7 @@ inputs:
     default: "600"
 
   idempotency-key:
-    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 the github.actor workflow expression to maintain one chat per triggering user."
+    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 the `github.actor` context value to maintain one chat per triggering user."
     required: false
 
   force-new-chat: