fix(action.yaml): drop github.actor example from idempotency-key description (#33)

GitHub Actions evaluates `${{ }}` expressions in `action.yaml` at action-load time, including inside `description` strings, so the literal `${{ github.actor }}` in the `idempotency-key` description fails the action loader with `Unrecognized named-value: 'github'` for every consumer of `main`.

Rewrites the example sentence in plain prose so no `${{ }}` literal survives. The action-load failure is only observable via GitHub Actions' own loader, so the correctness check is reviewer eyeball on `action.yaml` line 62.

Failing consumer run: https://github.com/coder/coder/actions/runs/26046090719

🤖 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 `${{ github.actor }}` here if you want per-actor separation."
+    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."
     required: false
 
   force-new-chat: