feat(boards): ✨ implement azdo boards work-item delete command

[tmeckel]

Description

Implement azdo boards work-item delete per #269, mirroring az boards work-item delete. Deletes a single work item by ID; without --destroy the item moves to the Recycle Bin, with --destroy it is permanently removed.

Key decisions from the issue spec

• Project-scoped target [ORG:]PROJECT/ID parsed with util.ParseProjectTargetWithDefaultOrganization; positive-integer ID validation before any SDK call.
• Preflight ownership check: fetches the work item with GetWorkItem (fields limited to System.TeamProject) and verifies the project matches before prompting or deleting. Mismatch/missing project is an error and never calls DeleteWorkItem.
• Confirmation prompt (Are you sure you want to delete this work item?, escalated wording with --destroy) unless --yes; cancellation returns util.ErrCancel.
• JSON output passes the raw *workitemtracking.WorkItemDelete to the exporter (no view struct); plain output prints Deleted work item <id> / Permanently deleted work item <id>.
• Uses the vendored workitemtracking.Client.DeleteWorkItem; no new mocks needed.
• New shared helper BelongsToProject + TeamProjectField in internal/cmd/boards/workitem/shared for reuse by sibling work-item commands.

Files changed

• internal/cmd/boards/workitem/delete/delete.go — new command
• internal/cmd/boards/workitem/delete/delete_test.go — table-driven tests covering success, destroy, confirmation, cancel, non-interactive, ID validation, project mismatch, API errors, and JSON output
• internal/cmd/boards/workitem/shared/shared.go (+ shared_test.go) — BelongsToProject ownership check
• internal/cmd/boards/workitem/workitem.go — command registration
• docs/ — regenerated CLI docs

Verification

• go test ./internal/cmd/boards/workitem/... — 128 tests pass
• golangci-lint run internal/cmd/boards/workitem/... — clean

Closes #269