Add Timer sample for monthly subscription

[sachinsharma3191]

Summary

• Adds a Timer sample implementing a monthly subscription workflow with cancellation handling (closes Sample request: Timer #18)
• Uses Temporalio::Workflow.sleep for 30-day intervals and Temporalio::Workflow.execute_activity to charge the user
• Handles Temporalio::Error::CanceledError for graceful workflow cancellation
• Follows the .NET Timer sample reference pattern

Files

• timer/subscription_workflow.rb — Subscription workflow with sleep loop and cancellation
• timer/my_activities.rb — Simple Charge activity
• timer/worker.rb — Worker registering workflow and activity
• timer/starter.rb — Starts the subscription workflow
• timer/README.md — Usage instructions
• test/timer/subscription_workflow_test.rb — Time-skipping test verifying charge and cancellation
• README.md — Added timer to the samples list

Test plan

• Rubocop passes with no offenses
• Time-skipping test passes (1 run, 2 assertions, 0 failures)

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