chore: add Varlock environment schema for secure secret management

Adds .env.schema with LINEAR_API_KEY (required, sensitive) and
LINEAR_DEFAULT_INITIATIVE_ID (optional) definitions. Enables
`varlock load` validation and `varlock run` for safe secret injection
without exposing keys in terminal output or Claude's context.

Co-Authored-By: claude-flow <ruv@ruv.net>

Author: wrsmith108

.env.schema

@@ -0,0 +1,10 @@
+# Linear Claude Skill - Environment Schema
+# @defaultSensitive=true @defaultRequired=infer
+
+# Linear API Key - Required for all operations
+# @type=string(startsWith=lin_api_) @required @sensitive
+LINEAR_API_KEY=
+
+# Default initiative UUID for createProjectWithDefaults()
+# @type=string @required=false @sensitive=false
+LINEAR_DEFAULT_INITIATIVE_ID=