fix(cli): restore -y shorthand on todo task delete and skill setup

[jackjin1997]

Summary

• What changed? Registered the -y shorthand on the local --yes flags of dws todo task delete (internal/helpers/todo.go) and dws skill setup (internal/app/skill_setup.go) — Bool("yes") → BoolP("yes", "y", ...).
• Why? Both subcommands declare a local --yes flag with the same name as the global persistent --yes/-y. During cobra's persistent-flag merge, the same-named parent flag (and its -y shorthand) is skipped, so -y resolved to nothing:

  $ dws todo task delete --task-id <id> -y
  { "error": { "message": "unknown shorthand flag: 'y' in -y" } }
  

  Only the long --yes worked, which is surprising for an AI-agent-oriented CLI where -y is the documented global shorthand.

Fixes #370

Verification

• make build
• make lint — go vet passes; staticcheck not installed in my local env (CI covers it)
• go test ./internal/helpers/ ./internal/app/ (incl. new TestTodoTaskDeleteHasYesShorthand, TestSkillSetupHasYesShorthand)
• make policy
• ./scripts/policy/check-generated-drift.sh
• ./scripts/policy/check-command-surface.sh --strict

Notes

• Why add the shorthand instead of removing the local flags (the other option in the issue): removing them would also drop the shorthand and would break a standalone unit test (TestRunSkillSetupRejectsSkillFlagInMonoMode) that constructs skill setup without the root persistent flags and passes --yes. Adding -y is the smaller, safer change that directly fixes the reported symptom and keeps both standalone and mounted usage working — cobra's merge skips the duplicate-named persistent flag, so the local -y becomes the active shorthand with no collision.
• RunE reads via cmd.Flags().GetBool("yes") in both commands, so behavior is unchanged beyond the shorthand now resolving.