feat: add coder secret import for bulk secret files (#27534)

Adds `coder secret import <file>` to bulk-import dotenv, JSON, or YAML
secrets through the existing batch API. The command infers the format
from the extension or accepts `--input-format`, supports non-interactive
stdin, validates files locally before upload, and warns when imported
keys cannot be injected as environment variables.

Reviewed and updated by Coder Agents on behalf of @dylanhuff-at-coder.

docs/reference/cli/secret.md

@@ -24,6 +24,10 @@ coder secret
 
      $ echo -n "$NEW_SECRET_VALUE" | coder secret update api-key --description "Rotated API key" --env API_KEY --file "~/.api-key"
 
+  - Import secrets from a file:
+
+     $ coder secret import ./secrets.env
+
   - List your secrets:
 
      $ coder secret list
@@ -43,6 +47,7 @@ coder secret
 |---------------------------------------------|---------------------------------------------------|
 | [<code>create</code>](./secret_create.md)   | Create a secret                                   |
 | [<code>update</code>](./secret_update.md)   | Update a secret                                   |
+| [<code>import</code>](./secret_import.md)   | Import secrets from a file                        |
 | [<code>enable</code>](./secret_enable.md)   | Enable a secret so it is injected into workspaces |
 | [<code>disable</code>](./secret_disable.md) | Disable a secret without removing it              |
 | [<code>list</code>](./secret_list.md)       | List secrets, or show one by name                 |