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.

cli/testdata/coder_secret_--help.golden

@@ -17,6 +17,10 @@ USAGE:
        $ 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
@@ -34,6 +38,7 @@ SUBCOMMANDS:
     delete     Delete a secret
     disable    Disable a secret without removing it
     enable     Enable a secret so it is injected into workspaces
+    import     Import secrets from a file
     list       List secrets, or show one by name
     update     Update a secret
 

cli/testdata/coder_secret_import_--help.golden

@@ -0,0 +1,19 @@
+coder v0.0.0-devel
+
+USAGE:
+  coder secret import [flags] <file>
+
+  Import secrets from a file
+
+  Every key in the file becomes a secret. Keys allowed as environment variable
+  names are injected into workspaces under the same name. The import is all or
+  nothing, and existing secrets are never overwritten. Pass - to read the file
+  from non-interactive stdin (pipe or redirect).
+
+OPTIONS:
+      --input-format env|json|yaml
+          Format of the secrets file. Inferred from the file extension when
+          unset, and required when reading from stdin.
+
+———
+Run `coder --help` for a list of global options.